Are you talking about the free RP you get from leveling? Then sure you can buy a on sale champion that costs 395 RP, it'll save you some IP. But only if you aren't interested in buying RP to begin with, which you shouldn't, but if you are considering buying RP and looking for stuff to buy then do not buy champions with it, its a waste of RP.
Unless you care more about time then money, then you can buy champions with RP and use IP to buy runes. But you'd be wasting your money, think of the games you can buy instead of unlocking stuff you can unlock with time anyway, think of the games you can buy instead of overpriced bits of pixels.
But whatever, do what you like and have fun while it lasts.