You can chose between a few champions, but 400 RP wont get you far:Mmm... I'm level 19 right now.
Could I receive other RP?
Are 400 RP enough to buy something?
http://leagueoflegends.wikia.com/wiki/Category:260_RP_Champion
Most of their sales put champions/skins at 487 RP. It's a smart move by Riot because you'd still have to invest in RP if you want any of those. You can buy cheap skins and champions, but you will have a lot of spare RP after that. You get the same "problem" when you buy RP from the store. You can't just buy RP for a champion. You have to buy fixed amounts (different in EU and NA) which leaves you with a lot of spare RP - almost enough for another champion.