Here's my suggestion for those of you who don't have any runes yet:
Wait until level 20 before you buy any with your IP.
Once at 20, you can start saving for your runes. I'd suggest getting them in a couple orders/groups.
1. AP or AD quints. You'll want a set of each (3075 IP for each set of 3).
2. Scaling MR Glyphs (x9 = 1845 IP) If you play a lot of ARAM I'd recommend these in particular.
3. Armor Seals. These can be a mix of flat and scaling. If you go for a mix, I'd recommend 6 flat and 3 scaling.
4. Runes - if you want to go cheap and just have 1 set for now, get the Hybrid Penetration ones (Double Penetration would be a much better name). If you find yourself really playing a specific role, this is where you can start varying things a bit.
Lastly, if you really like to play support, then you will eventually want to get the gold/10 seconds quints.