Consider selling to Internet Brands (http://www.internetbrands.com/). They look for sites with healthy organic traffic and the possibility to improve monetization. So those 500K visitors per month should come from a healthy list of keywords indexed in the major search engines and a high page rank.

Does the $5K per month come from ads, affiliate links, product sales, subscriptions? Most likely it's just AdSense, so they'd love that, since they can swap those out for direct sell ads that can be sold across multiple sites they own for a yield 20x as much as you could get from AdSense.

Is the content 100% unique? Who wrote it, and how fast is new content being added to the site? Do you have a staff of writers who are writing quality content? Or is the content user-generated? They'll want to know how much it will cost to keep the site going long term.

Typically a site this size, assuming the traffic is healthy and targeted and the revenue looks like it will keep coming in should go for 12-36x monthly earnings. By Internet Brands standards, 1000 articles is not that much, the traffic is decent, but the revenue is on the low side. Compare that to a site they own like Steve's Digicams, which has 20,000 high quality camera reviews spanning 12 years, a PR4, and revenue in the mid six figures (probably low six figures when they bought it), and for which they likely paid $1 million.