My guess is the IRS will follow policies ideas they will borrow from the stock market. The IRS could 1) Treat forks like stock splits, 2)Treat forks like a merger or 3) Treat forks like a reverse merger
1) Treat forks like stock splits - When a stock splits you get a ratio (usually 2:1). If you held 100 shares prior to the split, now you have 200 shares and your cost-basis remains the same. This makes since to me as a way to calculate the cost-basis of PoS coins or tokens. It works the same as dividends paid in stock. Crypto Example: You own 400 "XYZ" Coins with a basis of $250 per coin (total basis of $100,000). You earned 40 additional XYZ coin from staking and now own a total of 440 XYZ. Your total basis is unchanged, so your basis per share is now $100,000 divided by 440, or $227.28. Clearly, this doesn't make sense for a fork, since there is a "new" coin created. But its important to understand stock splits to understand mergers.
2)Treat forks like a merger - Imagine you own company/coin A and it merges with company/coin B and all your company/coin A shares become company/coin B shares. You can look up what the terms below mean and how to calculate them, but for the purpose of discussing forks the details will bog everything down too much.
To calculate your cost basis you need to know:
a) Your original cost basis for company/Coin A
b) The taxable gain (or loss) "realized" in the merger/fork taking into account any cash received No forks to date have had a cash payout
c) The taxable gain (or loss) recognized in the merger/fork
d) The total basis in the new company B stock/coin
e) The gain/loss on fractional shares
But here we have a problem too. A "merger" isn't happening when we are looking at forks. The action looks most closely like a reverse merger, which is much rarer and is one of the most common ways for large companies to defer taxes. It may be possible to defer taxes using strategies similar to reverse merger tax strategies.
3) Treat forks like a reverse merger - I actually ran out of time researching this one because reverse mergers are so convoluted (to me anyway). I'll be back with an edit soon! Stay tuned!