Do you mean how much does the certification cost ? Well, if it is certified, the valuer will already would place a value to your domain name. https://www.godaddy.com/gdshop/dna/appraisal.asp?ci=4370 In the end, the value is really depends on how much the buyer thinks it is worth. The market trend is towards dictionary words and short domains names with good PR. It might change over time.
Are you talking about certified domains by Premiumdomians.com??? I have certified premium ones from them. Your question is too general. All depends on the ccTLD & what the words are. Could be a couple of hundred of dollars all the way to a few million dollars.
In addition, there is a common misconception in the belief that any 3-4 letters/numbers names are premium domains when only a few really are.
I'd like to know this, too. What if I have an 11 character (2 generic words) .com premium domain (certified by premiumdomain.com) that has let's say 500 score (of 1 - 1000 premiumdomain.com range). How much is such a domain worth? I think each premium domain is worth at least $X,XXX.
Don't fall for any automated domain appraisal system. You need a real person's opinion. A domain is only worth a good amount if there is a buyer out there willing to spend that much for it. Everything depends on what your 2 generic words are, and whether there would be a demand for such a domain.
Worth as much as someone is willing to pay. If your buyer has motivation and you can get a greta price for premium domains.