When you pay to propose a compel, the point you paid returns to the GM's infinite pool of fate points. The proposed compel is then negotiated with the target of the compel. Once the compel is agreed, a fate point is paid from the infinite pool of fate points to the target player. If the target player rejects the compel, their fate point is also paid to the infinite pool of fate points.

If this were not the rule, you would be able to propose compels risk-free to drain other people's fate points.

Fred Hicks, the publisher and one of the main developers of Fate Core, agrees with this interpretation (in a comment to a now deleted answer):

The GM always runs the compel; FPs spent and gained go into or come out from the GM's infinite pool for compels. The proposer never gets a payout.

This has now been added as a clarification on the Fate SRD.

In Fate Core, you the GM wield two separate but equally important threats: numbers and the plot.

So, a small prelude: Fate Core PCs are proactive people, capable of taking dramatic action. On some level you kind of want your PCs to be getting into trouble with compels and hitting their aspects to get out of it, deciding to take failures to save Fate Points for later. A common antipattern some people find themselves in is burning Fate Points in order to avoid trivial consequences and not having them when it's time to make the real fireworks happen.

I get that things have gone a bit too far the other way - just be careful not to overcorrect.

Adventure Fractals, or: How Big Was My Number

A quick big number prelude - are you keeping up with your party? I imagine if their refresh has gone up they've also put quite a few points into their skills. Is anybody topping out at +5? +6? Have you gone through the 5/11/18 significant milestones it'd take to blow their cap but everybody's just chonking out at +4?

It may be time to consider elevating the baseline opposition from "based around +4" to "based around +5". Or even higher! Because, to hook the plot portion of things in, your PCs have already been through so much and they're moving on to bigger things.

When you have obtained a number of appropriate bigness, it's time to consider the adventure fractal. (While the concept is setting-agnostic, the SRD is its implementation in Fate of Agaptus -- just ignore that little line about what happens on a +9.) Basically, of the four adventure nucleuses of Combat, Exploration, Interaction, and Lore, pick one to be the adventure focus at +1 to big number, one to be backburner at -3 to big number, and the other two are at -1 to big number. When the appropriate thing comes up in the adventure, base it there; if "the appropriate thing" is a higher-test NPC bump them up as usual.

To hook the plot portion in again, if something is backburner to the adventure then it's actually backburner to the adventure; you can't solve the adventure that way. If you're doing an adventure where exploration takes the lead and interaction is on the backburner, like finding an ancient secret buried in a forgotten ruin, then you can't use interaction to get to that secret directly. It can still come up in the more preparatory phase of the adventure when you're securing resources, but the invokes from those advantages are going to get bled off in the harder exploration segments (with incidental combat and lore).

Plot Entanglements, or: As Compels Do

So, compels are different from adverse invokes in that invokes pump up numbers while compels work to shift the plot. The easiest way to make sure your compels have some teeth in them is to make sure you're always going from can to could, or from could to can't.

Or, as a longer form: there are things your character can do, no roll, like walk down the street; things your character can't do, no roll, like jump to the moon; and things your character could do if they put up a big enough number, like hurtle an oncoming motorcycle. When a compel happens, it's introducing doubt into something that seemed straightforward -- moving from can to could -- or closing off an avenue that seemed possible -- moving from could to can't. Compels are there to start a scene, a place where the PCs can make rolls to effect, or to end one in the GM's favor.

But again, if people are willingly taking compels early on to build up a stock of Fate Points to make use of when things get serious, to some extent that's just kind of how a story goes -- you get up the tree in Act 1, down in Act 3, and in between you get rocks thrown at you. Even when you're making compels mean something early on, don't feel the need to go too big - what's left for the finale then?