Ballpark figure would be around last 12 months of site generated income + your additional work aka introduction month or two after a sale with the new owner. But I wouldn't sell it if it makes that kind of money with so little effort. I would rinse and repeat few more sites of this kind and/or build a little more network around it. But that just me.
If it works and brings you steady quite good income why sell it. That is probably one of many questions from a possible buyer. Like HW said be prepared to show all details year or two back at least.
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