bug with prices in the store ?

I just bought an extension for 50 more pokemons, now 750, and it was less expensive than usual.
I had 405 pokecoins and still 270 after. Si I payed 135 only, and I had no pokemons backing from arena during this time, nor before or after...
I didn't notice something about that before, and it's still written 200 on the store.

Problem is that everybody here is yet up To 1000, so nobody can test what I say loool