I'd say find an establishment that has a Gemologist on site or works there. Much easier to know what you are paying for.
I purchased my first engagement ring from a well-known Jeweler that I will not mention and paid $9750. Very nice looking ring: designer band & pedestal, 24K gold, a little over a 1 kt diamond, .5 kt baggets per side, with a nicely placed ruby. I took it to a gemologist to have it appraised for insurance purposes and found it to be worth $5800. (Turns out diamond was flawed and not same grade as displayed and baggets were loose and flawed. Only real advertised piece was the 24k gold.) I nearly *crapped in pants* and started to get infuriated until I saw my fiancee heart broken so I held back. We took the ring back and went to the chain owner's office where he taped our discussion so he wouldn't get his rear end sued.
2nd attempt: Bought the final engagement ring of a widely known maker (custom done) with a far superior main diamond, band diamonds, high grade rubys, and platinum (than the last ring) for what I thought was going to total $15K for ~ $9K (not including the extra wedding band). I had it appraised and was within $50 of marked value. So, my fiancee and I custom did my wedding ring as well.
Oh, and we dated for 10 yrs before getting married.
Bottom line: (for those of you who didn't want to read all that) Is have it appraised by a known gemologist. They will know the value of the ring. I would not rely on purchasing it from a store who says they can appraise it for you without a certified gemologist. (Check for diploma - I've seen them at nearly all gemologists' sites)
-Regards
Majority of the time the biggest cost will be the main stone if it is a diamond. And it depends on clarity, inclusion, size, etc..