well in the new article on the comunity blog there is a screen shot and the 4th level of one of the XP perks is 100,000c for a 10% boost (that stacks with the other 3 levels) so i would guess it's 100,000 per 10% upto a maximum of 500,000c and a 50% boost! Of course that might just be placeholder prices and values. i guess we'll find out next Tuesday