A mix of rank and how old it is with rare skins and such. I've had the account since Season 1 so it had a lot of rare stuff on it. The account was diamond ranked so that adds anywhere from 100-200$ to the amount. I charged 100 for the skins basically.
If I had cared to get it to master it would have been worth 500+. I can't stand selling account anymore though. The time and effort really sucks for boosting people and selling accounts. It makes you hate the game. I'm moving to another game now so I decided to sell it because Riot made changes to the ranked mode that I dislike and a few other games show some promise for esports/moneymaking.