rp got robbed?

hey, rioters or fellow league players. So, i bought 10 dollars of rp which is about 1300 , and i bought nightblade irelia skin. which is i think 520. Either way, it shows that i now have 512. Also keep in mind i had like 100 rp b4 rp purchase. i know im not being robbed from riot games, im sure this is some bug, but some understanding would help. Or am i being robbed? XD thank you sooo much.