In a nutshell, it's pretty hard to just plug in a valuation formula without knowing all of the information. I think a lot depends on the source of revenue and the type of name. Is the traffic coming from links on the web or typeins? Is the domain generic or a typo of a trademarked domain? Is it a seasonal domain? Are the earnings consistent over time or just a recent spike? Is it one click per month for $3 to $5 each... or many clicks with a lower RPC ?
These are all questions the buyer will ask and will base his purchase on.
Generally, domainers would buy a typo or TM sensitive domain for around 18 months revenue, assuming the traffic will continue. If its a TM risk free domain, but the source of traffic is external links, then buyers would have to assess the likelihood of those links continuing to exist - and generally, this would keep most from paying for more than just a handful of monthly expected earnings (3-6 months). If its a generic, quality domain with type-in traffic, you could anywhere from 5 to 30 years PPC revenue. But chances are if the domain is a beauty, its intrinsic value would outweigh the parking income anyway.
Keep in mind when selling for X months/years, you might be selling yourself short. If you do not have the domain optimized the best, or are using a parking provider that is underperforming for a certain keyword, you might be selling a domain based on $5/month revenue but the buyer will be able to turn it into $20/month. There are other things to consider, too. For example, someone could have a SedoPro account with a 30% overall bonus, while you are using a vanilla Sedo retail account.
One way to know for sure what you might get for selling the domain is to post a sales thread asking for offers. Provide all of the information (screenshots) and see what people are willing to offer you. If you are happy with the highest bid, take it.