Too bad this doesn't factor for Steam keys purchased elsewhere, if only to tell you what their cost would have been had they been purchased from Steam itself.
I have over 1300 items in my account but so many of them are items I would never have bought for any significant sum by themselves. Usually, they were one of many items in a bundle where a single item was the sole interest and regarded as worth the price of the whole bundle. Then there are the dozens of OSTs that I'd never have paid additionally for but make my account look like I've been on an insane spending spree. As far as I can tell, I've spent no more than around $250 to date for that absurd number of items, from all sources.
A good example was the bundle that first brought me to IndieGala. It included the PC version of 3D platformer Ty the Tasmanian Tiger, which I'd enjoyed quite a lot in the PS2/Xbox/GameCube era. The bundle was something like a dozen games for $5, which I would have gladly paid for Ty alone.
For added absurdity, those 1300+ plus items in my account don't include the 100+ duplicates I gave away, mostly to my niece.
Getting an exact count of all of this would be interesting but probably not interesting enough to sort through all the email receipts and add it up.