Speaking for myself, I always bought books when they had a 20% off sale so my average purchase was the same. I had 1 purchase over that (Dragonlance board game bundle) and 1 below (PHB for 50% off from the Essentials Set coupon). $25-30 is probably a good estimate to work with though.
Now what I’d be curious to know is how the shift to digital has impacted the number of individual buyers. For my table of 7, all of us owned a physical PHB and Xanathar’s. Once I started buying DDB books and sharing them, I became the only one buying books. Pre-DDB, that would have likely been 4 or 5 copies of Tasha’s sold that ended up only being one (at a higher margin, plus the subscription income). To the best of my knowledge, none of the other players bought any physical books at all once we shifted to digital. A master tier subscription is what? $50-60 so 2 purchases using the above estimates?
This is obviously all anecdotal, but I wonder how common the scenario above is for other tables.