If you're not min-maxing at the level cap, your best bet will generally be at least one, if not two, of the three Gathering professions, Skinning, Mining, and Herbalism. If you do decide to take a crafting profession, choose the gathering profession that meshes most closely with it (i.e., if you take Leatherworking, grab Skinning, Engineering, grab Mining, or Alchemy or Inscription, take Herbalism.)
If you want to make money using a crafting profession, the best choice before the cap will be Inscription, as the Glyphs you can produce as early as level 5 will be used even by level 80 characters, providing you with a constant market.
If you want to produce things you yourself will use on a regular basis, Engineering provides fun toys, and trinkets at much lower levels than they'll be availiable as drops. Leatherworking allows you to make your own armor, but drops and quest rewards will, for the most part, be better than what you can craft - at least until the Cataclysm expansion releases. Alchemy will allow you to make a variety of useful consumables, but in general, you will have to make many more than you'll be able to use as you level, and there won't be much of a market for what you craft.
Enchanting is a viable option as well, and you'll be able to enchant your own gear as you level, which is handy.
Realistically, until the +Stat bonuses at 400+ skill come into play, there really isn't a 'best' or 'most useful' profession. There are a few that will be much less useful to you as a hunter, such as Blacksmithing and Tailoring, but for the most part, you should be fine picking out whatever you find fun.
The only other major note worth making is that each of the three gathering professions provides a free 'passive' boost as well. Mining will give you some extra Stamina for free, Skinning will give your character increased critical strike chance, and Herbalism will teach you Lifeblood, a free Heal-Over-Time spell you can cast on yourself on a short cooldown. The crit from Skinning especially should be very handy for a low level hunter, though the other two perks are nothing to sneeze at either.
The system was changed, it used to be 1g, 5g, 10g, 25g, 50g maximum (I think), then decayed 5g a month to a minimum of 10g.
Now with the new patch, the respec cost is fixed to your level. At level 80, the respec cost is always 52g.
Best Answer
When using an auctioning Addon, you can always do an entire scan of the servers market. then you should have the current price for all items you can craft.
That way you don't have to re-scan every time you select that item.
Prices fluctuate over the weekend, after Raid id resets is the best time to sell enchants, because most people get new gear on those days.
Same rule applies to potions/flasks etc.
As for Auctionator, give Auctioneer a try, i find it a bit clearer and the entire scan is quicker. Also in auctioneer you can create a "shopping list" with all the items you want to craft and do 1 scan over all those items.