How much should I sell this runescape account for?

If you sell it with PHYSICAL cash, I wouldn't do it, you can get into big trouble. If you are going to sell with RUNESCAPE money, you'd be lucky to sell it because "selling" Runescape accounts is a big scam thats been going around lately, don't forget the 35 noobs that will report you for "scamming" and Jagex doesn't know if you're serious or not so, I would suggest just giving it to a friend to be on the safe side.