If you know how the diamond in your wedding set grades in terms of color and clarity, and what grades you're willing to accept in a larger stone, you might get some idea by looking on ebay for prices for similar stones.

According to this site (http://www.pricescope.com/), you can expect to pay about $4600 for a very yellow stone up to almost $28,000 for a really high quality stone. However, let me just add this. In my opinion (and that of my gemology teacher) anything over about 1 carat is getting a bit big for a ring. Personally, if find rings with bigger stones to be rather gaudy. Something in the size range about which you are asking is more appropriate in something like a pendant or broach, but it is your choice, of course. Oh, and one other thing. The above prices are for natural diamonds. Personally, I prefer to go with synthetic. With a synthetic, you don't have to worry about possibly supporting some bloody guerrilla army or the de Beers cartel and you can get a much, much better price on your diamond. ------------- Addendum: I edited this answer to remove the link to a site that I thought was selling synthetic diamonds. I found the site by doing an internet search for synthetic diamonds and the entry for this site said, "Flawless Synthetic Diamonds..." However, as I looked closer at the properties that they reported for their gemstones, I realized that there was no way they could be diamonds. Further examination of their website revealed that what they sell are diamond simulants rather than synthetic diamonds. Thus I took the website down, lest it continue to give the wrong impression as to what the person is actually purchasing.