Or you could just farm 1350 IP from playing some matches and save the RP you'd use to buy
for somebody else? Really, I'd argue that with any champ below the 790 RP/3150 IP mark it'd be a waste to buy them with RP, you could easily get the IP for 1350 champs and below in only a couple of days, at the most.
Please don't take that comment the wrong way, I'm not the one in control of your account, and you're free to use the RP you own as you choose, but I'm just trying to give a friendly suggestion on when and where it would be best to spend your RP. Heck, for only 5 RP more than Teemo's normal price (585 RP), you can get TWO rune pages right now, and I'd personally argue that takes precedence to the umpteenth degree over buying Teemo or any other 1350 IP/585 RP champ. (I bought RP recently just to get the last 10 rune pages for my book with the 2-for-1 bundle.)
Again, it's your decision, I just thought I'd give my two cents and offer a suggestion. Sorry if the wording seems a little funny or it comes off the wrong way.