On the basis of it making consistently around £4200 per year, and if the domain is of good quality with a large scale appeal to potential buyers, then once overheads are deducted, you could be looking at 2x-3x annual profit; it really depends on the possible scope and longevity if continued to be run as now.
2-3X is a good starting point but the biggest variables are going to be the product and market. A three letter domain is generally worth a few hundred alone.
Some good suggestions above. To be realistic, you have to figure "time" as a cost at a decent hourly rate. If the buyer has to spend an hour a week on it (higher than you said but just to illustrate my point) then that lost hour is "worth" something so has to be deducted from the perceived profitability before calculating a multiple for the actual value. And if you're selling to someone who has a high opinion of their free time (and therefore places a high value on the "burden"'of being effectively forced to give X amount of it up to keep the site they bought's revenue stream coming) then they may see no value in it at all.
By contrast, a parked domain which brought in £300 a month (still possible especially in the .com world) would have zero time overhead so no adjustment needed.
Being a miserable git, although attractive as it it sounds I wouldn't pay more than very low ££££ for it (that's based on it being a decent LLL too).
Even then, anyone with half a brain would want to see the stats and scrutinize them in detail. Presumably December was a boom due to xmas.
So we have £300pm adense which is nice, but having run adsense for over a decade those earnings can easily slide (maybe they're beginning to a bit now )
In fact a few of my good earners did three years then begin to slide, with nowt you can do to really stop it - even though your traffic/site remains constant and follows guidelines, so there is that to consider.
I've always found it very difficult to factor in future adsense earnings as *value* in a domain sale. There is some value though to be sure.
I’d say if any potential new buyers had the knowledge to keep it making the income then why would they buy it in the first place?
Incomes not attractive to those that would be able to afford it ? With selling price for those that are attracted to the income it tends to be too much for them no matter what you price it lol ? I’d buy other out and keep