I was looking through my old set of cards and found a Tranquility foil promo from Core Set 7 and have no clue how much it's worth( I know it doesn't have much value as a normal card). Is there a general rule as to how much more promos are worth than their base price? Or does it depend on the card?

http://sales.starcitygames.com/carddisplay.php?product=6653

I'm not really sure, think it depends on the card. Some are much rarer than others, while others aren't worth much at all. Can be hard to tell, as very few databases have all of them included. Really comes down to searchign around and seeing if anyone else is selling them.

Usually, foils are only worth much more than the non-foil if they are a valuable & commonly used card.
If they are just commons, people may just give a buck for them if they want them for a deck.
For example, a little while ago I traded a foil Thraximundar for about $25 (normal $9-10), and at an FNM someone wanted a foil Creepy Doll and gave me a dollar (normally 40c).
I buy lots of packs and have a bunch of foils of common foils, and I keep them in my inventory in the tiny chance someone wants them for a deck.

I use http://www.magicarsenal.com/ for checking foil prices (it's also very useful for looking for the most valuable cards in a single set)