You've run into one of the dangers of pre-planning a plot. I'll give some ideas at the end about how to plan campaigns so this doesn't happen as much in the future, but first we have to deal with the current situation. Other answers have dealt nicely with the "stay on the rails" and "take a short detour" options, so I'd like to talk about a third choice:
Take a new path through the bush. Forget the plot you had lined up. You've got interesting people and conflicts already present in the world and waiting in the wings, but you can let go of exactly how you expect it to play out. Go along with the PCs' choices and look for opportunities to introduce the interesting people and ideas you have prepared. Instead of killing off this NPC and negating the PCs' hard work, use the NPC as a gateway to new adventures which will incorporate your ideas in new, interesting ways.
It shouldn't be too hard for, e.g., someone connected with the NPC to kick the bucket in a ghost-inducing way, so the NPC drags the party in for the ride and you get a ghost investigation on the party's terms rather than on your own. Players tend to be more engaged with plots that arise from PC agency than plots which are thrust upon them.
Now lets talk briefly about avoiding this kind of situation in the future. For me, the key lies not in how I plan, but in what I plan. Instead of creating interesting stories to walk my players through, I need to create interesting people, situations, and conflicts which are happening when the game starts.
My favourite kind of game prep is to set up a complex set of NPC/faction/world interactions and then watch my players roll through them like a lopsided bowling ball. This way, my players can engage with a world and have their choices matter because they're interacting with dynamic processes. As the party acts and makes choices, I'm free to have the world react: NPCs change their plans based on PC action, natural events occur when it's most dramatic, and so forth.
An RPG story is about the PCs, so I like to give them a chance to really make the world sit up and notice their choices. The best way I've found to do this is to avoid planning stories that hinge on the players making certain choices.
3k at level 8 is abysmally poor.
I can see why your PCs are clamouring for wealth, it's not vanity, Pathfinder is balanced around wealth being used for magic items for survival so they are going to be struggling to survive.
If they are point-buy-20 or higher they should have over 66'000gp by this point.
Things you are going to have to do to reduce the difficulty:
Limit high crit weapons
As you play the game longer and longer the chance of any player ever getting hit by a crit goes up to something like 80% (this is ever being hit, not any single chance). Also, critical damage can go up hugely as well. As you have been so stingy they cannot afford anything like a Buffering Cap you must never have weapons with 3x crit range and ideally no 19-20/2x crit range either. They aren't going to be able to take a critical hit and you have left them no opportunity to protect themselves.
Limit weapon damage
Because you haven't left wealth for things like Shawl of Life Keeping to give a buffer for general damage loss you need to dial things back a lot. If game guides would suggest enemies armed with greatsword (2d6), arm them with something like a Longsword (1d8) instead.
Makes things close yet environment to your PC's advantage
The utter lack of wealth of course is going to limit PCs ability to extend the range of their attacks, for example Rogue could never afford Assassin's Sight, so you're going to have to keep things a lot closer yet leave plenty of leeway for your players to disengage or use a height or cover advantage. They are going to need it.
Nerf enemies' stats
Your players should have been able to afford items to boost their core stats by now but obviously cannot. So knock a few points off enemy NPC strength and constitution. Or at least impose some limit like they've been fighting and are lower HP already or are suffering from a disease, tired from travelling, their weapons are damaged or are inherently intimidated by the PC's combat prowess (see sickened, fatigued, broken and shaken respectively).
Still, all is not lost, there's still a lot of very good cheap stuff out there that I can recommend that will be appreciated despite their low cost.
This very cheap item can be hugely appreciated should they be in fear of falling.
Ammunition can be enchanted in groups of 50, so individual units of ammunition would cost 1/50th of enchanting the lot. This can be a great way of cheaply handing out single use bonuses such as Bane Bolt/Arrow against a particular troubling enemy type.
Another cheap item of huge use IF you are a generous GM and give opportunity to use it such as flat roofs nearby that with contracting rope they could quickly get up onto.
Short of being able to boost their Constitution or have a magic item buffer their HP, this will have to do for the big fights. This allows them to stay conscious but staggered when they go to negative HP and Troll Oil stops the steady tick-tick loss of HP when reduced to sub-zero HP.
Very cheap item but gives a considerable buff to protect them from being grappled.
A cheap poison with a very useful effect in how since it requires two saves to end the effect it always works. If your PCs are worries about self poisoning then Blackfinger Paste can cover that concern.
Low CL Potions (50gp+)
There are many 1st level spells which are really good but hard to get out and are viable even at the absolute minimum caster level.
- Touch Of The Sea: can make up for inability to buff stats to be a good swimmer.
- Enlarge Person: is a 1 Round to cast but as little as a Standard action to imbibe
- Blend With Surroundings: great for your rogue when he gets into a pinch.
This is a really good way to give your players the edge in the long run, especially if you are unwilling to drop high value items.
Be generous with traits
This could be a houserule or just a favourable interpretation of the rules but one thing I do is allow players to get a new trait every level and ignore the limit on how they cannot have more than one trait per "list". Considering there are over 1000 traits and normally players can only pick two of them at the beginning of the game that makes this a rather untapped resource.
It can reflect how the PCs have been developing as characters in acquiring new traits. This can do a huge amount to help keep them viable despite such a paucity of wealth.
Best Answer
If they want to get the party back together so badly, come up with sidequests that will help them do so. It doesn't need to be something they go out of their way to accomplish. If they're separated by a geographic obstacle, like a mountain, drop hints about a tunnel shortcut that is potentially dangerous, but will reduce their travel time considerably. The sidequest doesn't actually need to help them get back together (like if they follow false rumors, or an NPC lies to get them to do something), but they will be more likely to follow leads in that direction. They seem to have a clear party goal at the moment. Work with that.
As for their habit of refusing rewards, there are a few ways of handling this. You could make the relevant NPCs a little pushier ("we insist, it would be a great dishonor to us if you left without payment"), or maybe just have word of their altruistic nature spread. They could become widely-known folk heroes. Merchants might give certain goods for free, taverns and inns might refuse payment, and local craftsmen might fall over themselves for the chance to provide services. Of course, this could also result in people taking advantage of the PCs' generosity, and nefarious groups might become concerned by the group of famous goody two-shoes adventurers. It would be a different dynamic than is usual, but could be made to work.
If this doesn't work for some reason, they could acquire what they need through different means. Simple items can be crafted, and rarer ones might be found in monster dens or in dungeons. Liquid wealth is less important in this edition, especially because magic and masterwork items are harder to come by.
If all else fails, make their lack of resources an increasing issue. Every town they visit saps their resources, and if they don't find a paying job soon, they won't have enough money for food or lodging. If they can't afford a place to stay, NPCs might start treating them like the dirty vagabonds they appear to be. Even the most altruistic of parties will eventually accept payment if they are desperate enough.