As someone who has been buying sites, and are always looking to sell, then I usual look at the last 12 months, but base my model on the last 3 months revenue/costs. A site producing consistent results month after month over a long period is alot more valuable than 6-9 months net, and that is talking as a buyer, and as seller then 18 months net is probably the sweet spot for something non-exclusive with consistent results.
What can add to the price is excellent domain names, content etc.
Good luck on the sale
