I started buying runes at lvl 1 with tier 1 runes every lvl till I got to lvl 30. After that I started buying tier 3 runes. The tier 1's will give you enough of a bonus and are cheap enough to not put a dent in your ip. When you get into more advanced lvl 30 play you'll want tier 3 runes but until then tier 1's are fine.
I never buy tier 2 runes since they only improve your page a very slight amount for a lot more ip.