Riot has set the price of RP so low in this game that the only RMT ("chinese farming") available farms rank rather than IP. In other words, it's more profitable to buy rp and spend it, than it is to buy an IP grinding service and spend the IP.
If you have some RL dollars then probably the best thing you can do is buy RP when it's on sale, then wait for rune pages to go on sale 50% off and buy them then. With your IP, stockpile your IP until tier 3 runes go on sale for 30% off (that's like a 42% gain in IP or something like that). I'm sitting on 10k IP right now, and there are a ton of runes that I want still, but when that sale hits it'll be worth 14,285 IP instead of 10,000, and I'll have just saved a week of grinding or whatever.
If you have a decent disposable income, buy IP boosts, and buy your champions with cash. Spend all your IP on runes. Unfortunately I don't fall into that category. :(
Realistically, buy 16 cheap (450/1350) champs to start into ranked, and then buy runes for those champs, before buying any more champs.
Riot needs money. Honest. Developers have children, houses, etc. Riot's strategy is to drain away as much IP as possible, so that people spend RP instead (on non-cosmetic things).