We only get so many Influence Points per game, and we have to buy our Riot Points with real money, so I took some time to figure out the best way to spend them.

This is also published on my blog (same content).

For the adepts of the tl;dr ...

I would use IP mostly for :

• runes
• Heroic and Epic champions
• Legendary Champions when I have all my runes

and RP for :

• 3 rune pages
• Legendary Champions
• per day IP boosts (if I plan to win more than 6 normal games in a single day, or 10 games in 3 days)

Champions cost range from 450 IP to 6300 IP or from 260 RP to 975 RP.
But the ratio for a Heroic champion is 1.73 RP/IP and for a Legendary champion at 6300 IP the ratio is 6.46 ! (source)

So I will buy Heroic and Epic champions with IP, and Legendary champions with RP.

I will assume next that a reasonnable conversion ratio is 3 IP for 1 RP.

1 rune page costs 6300 IP or 590 RP (ratio 10.7)
7 rune pages cost 7*6300 IP or 2600 RP (ratio 17)
(source)

So I will definitively buy rune pages with RP.

The 7 rune pages bundle is prices as 4.4 single rune pages (in RP). Since I think 5 total pages is enough (1 for each major role (AP mid, carry AD, support, top bruiser, tank jungler) I will only buy 3 additional rune pages for 1770 RP.

Per win IP boost

When playing normal games, an IP boosts gives about 144 additional IP per game. (source)
A 4 wins IP boosts, costing 290 RP thus brings about 576 IP for a ratio of 2.
A 10 wins IP boosts, costing 670 RP thus brings about 1440 IP for a ratio of 2.15.
Since this is much lower than 3, I prefer to spend my RP buying rune pages or Legendary champions than per win IP boosts.
The only situation I would buy these IP boosts is if I would really need to buy runes, since they can only be paid in IP.

Per day IP boost

They cost 290 RP for 1 day, 520 RP for 3 days, 1846 RP for 14 days.
In order to get a IP per RP ratio of 3, you need to get :

• 6 wins in 1 day
• 11 wins in 3 days (3.6 wins/day)
• 39 wins in 14 days (2.75 wins/day)

Since there is not much difference (about 3%), I will only buy what I need instead of committing 50€ to this game.