Need Mathematics Assistance

Hi,

I need a mathematics equation for an ELO service.

http://www.lol-academy.net/buy-elo/

I need an equation almost identical to that, maybe 5-10% cheaper. Anyone up for the task?

My site also has sliders for current and desired ELO, so just use variables for those. We'll call current ELO $x, and desired ELO $y, total ELO ($y - $x) will be $t.