I tend to value them at a % of the resale value but would depend on customer if a customer is going to build a fab site generate lots of interests etc (its like me letting you the land… you pay to build a house on it… pay me to live in it… then give me the house at the end of the lease…
Example if the domain was worth a one thousand pounds my return from bank would be 5 % happy to let at that price there are lots of other things devaluation, income from site already etc etc but as a starting figure I have found it generally works