How much is fair for three qualified leads?

Regardless of the job size, how much would you say is fair for three qualified leads with the expectation of one becoming a paid customer? The job requirement is already drafted, the customer is ready to pay, no back-and-forth required. You just need to review and accept or decline.

The tricky part is that these leads could be of any job size, but would be passed over months by months, so there is consistency there. I am more familiar with London prices, so if you don't mind mentioning your city or country, that would be much appreciated.