has anyone kept track of the purchases (ignoring sales during the pandemic) and calculated an average price per share?

Though certainly a useful number to know, unlike some holdings a simple average price per share isn't a great guide to how well they have done so far. I think in the case of Berkshire's investment, you'd want to include all the moving parts, which gets complicated very quickly.
* The dividends are not inconsequential since March 2022, so you'd also want to estimate the after-tax dividends received to date.
* Add the after-tax benefit of the 8% interest on the preferred to date
* The in-the-money mark-to-market profit on the warrants, less a tax provision
* The premium received on the fraction of preferred stock that was redeemed to date
* The time-weighted amount of money put at risk to date: e.g., some recent purchases are profitable despite tying up money for only a few days, and the cost to exercise the warrants has not yet been disbursed.

Messy.

Jim