I guess that really depends on where you are going to sell it to be honest. If you were to go for ebay or something like that then perhaps you could sell to an upwards of £35-£40 pounds with a couple of pounds per game or something like that but if you were wanting to sell it to a games company they will give nowt in comparison. Grainger Games give a max of £20. I'm guessing this will be similar for other companies either in the UK or the USA.
Don't get excited though as you won't get loads for it so you'll need to raise quite a bit more saying you want to get a new 3DS XL which retails in the UK at least for about £120ish or for the new version £180.
FFX, Diablo III, Lightning Returns (New Game+) FFXIV A Realm Reborn, AC4