I'm holding a 25mm moissanite that I bought for fun because it's the roughly the same size as the hope diamond and it was $250 without a certificate. There's a ton of competition now so prices have started dropping. A 10mm is ~3.9ct and the loose gem is < $100 on the first link. The rest is just the cost of the ring. Adding silicon to the carbon makes it slightly softer, but significantly less prone to cracking which was a common failure point.

Regarding OPs original question, it's very hard to verify things online. I'm barely a hobbyist so I can't make recommendations, but be aware a lot of glass is shipped from India. The best you can do is be sure to read reviews and try to do your due difference. A lot of gems can be grown in labs but if you are investing in hardware, you are going to grow diamonds (moissanite) because they have the highest value. If you do opt into a traditional ring, certificates for moissanite are not the same as diamonds. They are closer to receipts to verify what it is, and you typically want to see the certificate id engraved into the stone.

It still seems fucking insane to me to spend 2,5k on a ring but you do you.

Yes, my stone is a full carat because my husband wanted a round number I guess haha. Anyway, it does look rather opulent given our economic status, but I Iike it and it fit his budget.