GMG question...

base price

I'm pretty sure they've made their system so it can't be cheated. Would be pretty stupid of them to let you earn higher amount of credit than what you actually paid for. So you'll probably get less credit than what you paid.

I'd like to know if anyone knows for sure =p. I've never even used the trad-in feature on GMG.

A lot less, It's very rare for them to have a deal where you can gain credit, as most trade-in values are relatively low. There are some exceptions, however, when you can get a game for free and later trade it in for usually $4 or $5. To answer your question, maybe $2.50 and not much more.

The trade in value is written when you buy the game. Many (most) games can't be traded in, in particular Steam games. Others can be traded for low value. I bought Deus Ex for $2 (normal price $8) and it has a $1.50 trade in value, and that's pretty high compared to other games. For example Deus Ex Invisible War has a trade in value of $0.20 and so does Tomb Raider Anniversary.

It will say on the game page.