How much would you sell a used white iphone 3g 16GB?

My friend wants to buy my old phone and I'm not sure how much to sell it really. I don't want to charge them an over priced amount either
Its in really good condition for being a few yrs old with probably only 1-2 minor scratches
The only issues are that the home button needs to be pressed harder to work and the phone isn't updated to the recent iOS

IPhone 3g is pretty old. I'd say around $40-60

$50 not a penny over.