Save up as much as the regular IP price for each rune you want to purchase. I would estimate based on the maximum costs for each kind of Tier 3 rune: Seals, Marks, and Glyphs cost at most 820 IP, while Quintessences cost at most 2050 IP.
Seasonal runes fall in at Tier 2.5, which are at best as good as Tier 3. Because of this, seasonal runes cost at most as much as their Tier 3 equivalents, although historically they have cost less.
For example, the Quintessence of Frosty Fortitude available during Christmas cost 1550 IP compared to the regular 2050 IP - a notable 500 IP less for the same stats.
For some historical value, you can check out the list of runes to see what Riot has done so far, and in particular the list of Quintessences has the IP prices recorded.
Riot will likely never create a completely unique seasonal rune in terms of stats; players generally frown upon any game-changing mechanic that is only available to purchase for a limited time, and Riot has been very conscious of that.
Also, make sure you are at least level 20 in time for the event so that you have access to the highest tier runes. Lower-level characters are not able to purchase higher tier runes in advance, although Riot has more recently released Tier 1.5 runes to cater to lower-level players.