Well, I just checked one character to get some actual data. I have a level 45 who still has 15 SO's in his build and it said it would be 795K to upgrade.
If he had no IO's, he would be upgrading 83 SO's so that would cost 4.4M inf.
I normally upgrade SO's every 3rd or 4th level (i.e. waiting until they turn white or yellow). His are all still green. If I was using SO's to 50, then I'd have two upgrades on the way to 50 so that would cost about 10M. Figure about 12-14 upgrades on the way to 45 with the lower ones costing far less. I'd estimate a total cost in the range of 35-40M inf assuming you upgrade at the rate I do; every 3-4 levels.
And yes, there would be a huge cost difference upgrading every level. Each time you upgrade, you're buying a new set of +3 enhancements. Doing that every level would cost you approximately double compared to upgrade every other level. I would estimate upgrading every level would cost about 140-150M inf. HUGE waste of money, considering I have fully IO'd some characters for less than that.