You have to calculate the value of the site, because I don't believe the underlying domain has any value divorced from what's sitting on it. However, a site that's making a consistent £100 a month with no effort* must be worth a few hundred pounds at least. Well done on making something out of a fallow asset.
*if there is no effort involved. If you're actually having to work on the site on an ongoing basis, then you need to divide the £100 by the number of hours you worked to get an average hourly rate. It better be high, otherwise why would anyone want to take it off your hands? (for example, £100 a month but 20 hours of work is a sub-minimum wage £5/hour)