There are only three skins that can be bought for this price. You can also wait for a mid tier skin to come on sale.
Or you can buy a low IP cost champion.
Personally, I purchased more RP the week my main at the time, Tryndamere, had a sale on one of his skins, since 400 RP wasn't enough for that skin even on sale =P Haven't looked back since - I now own a skin for all but one of my 41 champions, and I usually buy champions I'm interested in when skins I like come on sale.
That 400 "free" RP doesn't do much on its own, other then provide an introduction for the awesomeness of RP purchases.