Revenue is about $6000 per month and the costs for running the site are probaly quite low, minus the costs for the owner himself, so maybe monthly profit is around $4000? So given the formula for non-internet companies which are valued around 2-3 times the annual profit, the company would be worth around $120000.

Additionally consider these facts: the site is quite popular and has gained some traction but is still primarily interesting for a specific niche (startup founders). I would say the price for acquisition is at least $500000 and may very well be around or above 1 million which is even more likely because the owner has agreed to continue work on the site and growing it even more.