Not sure if this has been mentioned, but a fun little side note:
If you had only 6300IP/975RP champions left to buy and didn't care which ones you got you could buy 3 mystery champion shards and reroll them for a guaranteed 6300 champ. Champ shards are 1700ip each so 1700X3=5100, 6300-5100=1200. So you'd be saving 1200ip per champ.
Or we could go with RP, shards are 195RP each, 6300 champs are 975RP each. So 195X3=585, 975-585=390. So you're saving 390RP per champ, $10=1380RP so 1380/390=~3.5 so you could buy 3 champs for $10 as opposed to 1 champ if you buy shards.
I count 32X6300IP/975RP champs (including Ivern after he goes down in price) so 32X1200=38400 or 32X390=12480 , so you'd save 38,400ip or 12,480rp doing this. You'd be saving at least $50(7200RP) by doing this method (I'm too lazy right now to figure out the actual amount you'd be saving since RP scaling is different between $50 payment and $100 payment).
Again though, using this method only guarantees a 6300 champ if you own all 4800 and lower champs. It's also random so you won't get a specific champ you want unless you're very lucky. I'm also unsure if there's a cap on how many shards you can buy per day or not. If there is then this method would take significantly longer than just buying all the champs for that extra money.
Hope this was informative!
-Vaz-