Hi Wout,
It's difficult to say without a complete list, Jet Force Gemini for example is fairly common whereas I've seen Mario Tennis sell for quite a bit.
I reckon you should just check out Ebay prices, work out an average of each game, add them up and that's probably a fair price. Maybe offer a little lower since you're buying in bulk and it saves him the effort of listing them on ebay. Ultimately though as long as you both agree a price that's fair for you two, then you'll both be happy.