The extent of the income mulitple is deduced from many factors, most hard to pinpoint such as with domain appraisals. The prospective owner is going to need to know how stable is the income and for how much longer they can realistically count on it continuing.
Significant factors in deciding a fair mulitple, in no particular order are;
Quality of domain. Domains are assets on the balance sheet.
Seasonal affect on income (is the income proven over 2 years to prove a proper average). How far can you go back with the income. It may help influence a buyer's willingness to reach a certain price.
Perceived longevity of the product or service. i.e. is is Playstation2 based or based on technology that may be made redundent.
Time and effort on maintenance should be taken into account, not just expenses.
Has it got a solid, growing or stagnating paid membership.
Is the site based on solid, imherent foundations. i.e. if you sell the site will 2000 links, leads and sponsorship suddenly disappear.
How much work and help are you willing to put into the transition of ownership.
Are you going to set up a rival site as soon as you sell the other one.
If you have all the answers to the above you can justifiably ask for much more than 2 years IMHO.
On the flipside, if you feel the market segment is slipping away then settle for a lot lower multiple.
