I agree with PokerMonger..if its already generating $5k a month and you needn’t change a thing…I would hold out for a minimum of 5 years return…or even hold it and wait.
Taking the following factors in mind.
What you need to consider is that with a bit more work what could it generate and also taking into account future natural growth in your price. Growth is a definate factor in price and it is much overlooked by those making offers…in the ppc rev world currently there are people paying 3-5-10 years rev on good names ..tm typos obviously aren’t worth that much due to risk factor.. generic typos are not an issue in most cases and the first rev pattern can be applied. Personally I think people in this realm tend not to look at the other value in a domain aside from the actual ppc…tho this is changing…people are starting to see the brandablity, the usability, the keyword value…and willing to pay over and above the ppc rev for domains.
Also is the buyer an enduser or a domainer…this is a crucial factor.
With casino affilite sites…the potential income is far higher in many cases than ppc as if you can attract quality traffic the signups far surpass ppc clicks. So take all that in consideration before you sell…don’t get greedy…be realisitc…and also be absolutly sure you want to sell..once its gone its gone 
imho
zoot