They should give you about $19
How Much Money Will Game Stop Give Me For Pokemon Platinum?
I'm guessing about 1/3 of the price they are selling it for, example- if they are selling a ds lite at the gamestop for 90$ they would give you about 30-40 dollars if you turned it in to them. Cash is 20% less than what you would get for store credit, so keep that in mind.
I got about 23 dollars and some change but that was last month.
200$