Billboards trade at around 10% to 20% cap rates. First, you need to model out the value of the sign at this range of cap rates. Then subtract the cost of building the sign. Then put a discount on the time, trouble and risk of building the sign. The remaining number is your target for the value of the ground lease and permit.
I'm no CPA, but you'll have to pay regular tax on such a sale -- not capital gains. If you've held the lease and permit for a year, you might talk to your CPA about it, but most of the time that would be impossible.