I looked a few years back after reading a bunch about how synthetics were cheaper in discussions like this. I did not find it to be notably true then. There was barely a discount for synthetic at all, the places I checked.

Ended up with moissanite, which was significantly cheaper than diamond, but still not, like, cheap if you care about it looking as diamond-like as possible, though I probably could have done as well with diamond buying “used” if I’d had the patience for it and more knowledge to be more confident I wasn’t getting scammed.