In my opinion it doesn't make sense to pay taxes on a fork until you sell it and realize the gains. Otherwise do you pay taxes a second time when you do sell it?
If you had to pay market value for the forked coins they wouldn't be taxed when you received them.
I'm aware that there's situations where a CGT event occurs even without selling your asset, but this shouldn't be one of them.
You receive crypto worth $X that you paid $0 for. Thus, you have $X in income and owe income taxes on that. If you paid $Y for the crypto, then you had $X-$Y in income (similar to in-the-money stock options). Your cost basis would be $Y in the $X of coins, so if you ever sell them, then your income is $Z (value at time of sale) less $Y in capital gains.