Dang, I have a few suggestions but I can't recall the name of my first one. There is a chain that specializes in high end CZ jewelry in the Boston area. I know there is one in the Burlington Mall, and I think they are in Copley too, as well as other area malls. They are very overpriced IMHO, but their stuff does look good. Depending on how long it might take to make a substitution, this might be a good choice as they truly look real, unlike the TV ones.

That said QVC and all the other TV networks aren't a bad price. I'd also check out HSN and Jewelry tv JTV. Especially the latter as they have some awesome sales. If you go that route I would make one strong suggestion. All of these have a 30 day return policy. I would take it to a jeweler (a good one) and see if it is a good setting to put a stone in. Some of their settings are very flimsy and would either break while resetting or might not be secure enough for a real stone when you get it. That is a serious concern to your idea. The TV channels often have the cheap prices by not having the best workmanship. I have lost stones from a few rings I bought when I shopped from them, so I am very careful of their jewelry now. That's why I say have a jeweler confirm it would securely hold a diamond, otherwise I think the risk is too high!

If you have any problems with that, you might want check out the technibond and various designer lines on HSN. It is gold plated and all fake. I am not saying to keep this and reset. But quite often you can find a lovely fake set at a very affordable price. You might want to use this for now and then just get a total new ring when you go for it. This leaves you the freedom to get bigger or smaller when the time comes too. (And no one has to know why she got a new one...hey rings get lost, stolen etc all the time!).

There is the Internation Gem & Jewelry show that comes to Marlborough and a few other halls in MA several times a year. Wish I saved it but one is coming soon--they just sent me a postcard but I chucked it since the timing didn't work but it's soon. You do have to pay to get in, but there are discount and free coupons from vendors. There are TONS of jewelers offering everything. Last time I went I got a CZ (but set in marcasite not a wedding style) that looks to be a 2 carat. It is one of the best CZ's I have seen and when you put it next to a diamond you just can't tell. It was under $10! They had all sorts of styles, not to mention real stones, most in silver settings. Silly pricing. I would have bought a lot more had we not been in a rush, as it took much sorting. I know several sold findings too, as that is what I was looking for (but for earrings). It's next week just looked it up! http://www.intergem.com/event/detail.cfm?showID=45

And you can roll the dice and look on Ebay. It is definately a risk BUT there are amazing deals out there. A few years ago I got a tanzanite cluster ring in gold, took it to my jeweler to be sized and to make sure it was what they said it was. We got to chatting and he said he's sell it in his store for at least $500 (and this was before tanzanite went through the roof). I paid about $60 for it. He was blown away. So I think it's worth checking out....I made sure to order from someone with perfect feedback and who had been online awhile. I'd bet there are folks who specialize in stuff like this on there.

My last suggestion is to look at all the above sites and any others you can think of but look for a different center stone. Those often are cheaper/affordable and you easily could get a CZ popped in for next to nothing or do it yourself. I think you might get a much higher quality setting than one with a CZ in it. Also remember you don't want to get too big a CZ as that screams fake. I'd suggest shooting for the 2ct or less range.

I'll try to find/remember the name of the store and will come back with any other info I can think of (have a couple things in mind but no time to look now).

Links;
may add to this, since I'm a shopping addict and you have me thinking lol.

From HSN in real gold;

http://www.hsn.com/cnt/prod/default.aspx?webp_id=3333696&sz=0&sf=J&attr=2&prev=hp!sf!2

http://www.hsn.com/cnt/sf/default.aspx?sf=j&attr=372&cm_re=LN*Style*Rings&prev=hp!sf!2!3682

or for total fake;

http://www.hsn.com/cnt/prod/default.aspx?webp_id=3240807&web_id=3240807&sf=j&attr=3010&prev=hp!sf!372!3010

At JTV (usually better quality never saw their CZ though);

http://www.jewelrytelevision.com/index.aspx?tid=14582&Nrppo=1&Ns=p_priority|1&Nrecs=48&N=51%20511%20239&No=0&ptid=7362&cm_sp=&cm_re=ProdRelatedItems*1*1&cm_pos=1&bca=BLC198&R=2538514

or for 14k CZ rings;

http://www.jewelrytelevision.com/index.aspx?Ntt=bella%20luce%2014k&cm_re=full*Product+Type*Rings&N=51+511&cm_sp=&tid=7362

Found these web store specializing in what you want while trying to google the name of the store I mentioned;

http://www.diamondnexuslabs.com/jewelry/index.php?cPath=63

nice clearance section

http://www.ziamond.com/

And there were lots more so you might want to google.

PHEW got this question under my skin....almost positive this is the store I mentioned (but either I was thinking Natick Mall or they closed Burlington). I find them pricey but what you wanted.

http://www.landaujewelry.com/

Hope some of this helps and you find just what is perfect for you both!

FAST EDIT!!!!!!!!!!!!!!!!!!!!!!
Hope you check in. I happened to be shopping on Ebay (for me not you lol--even won what I wanted at much less than I hoped for). Anyway in poking around I spotted this & amazingly thought of you right away. Check it out, and if you miss this one they have tons more. (including a few things I'm not considering lol). But this looks lovely, and the stuff is beyond resonable IMHO. I'd look at this and others by this seller.

http://cgi.ebay.com/2-54-Carat-Briolite-Simulated-Diamond-Ring-NO-RESERVE_W0QQitemZ250111723184QQihZ015QQcategoryZ92902QQtcZphotoQQcmdZViewItem