I remember looking at engagement rings about 5 years ago, my now wife is quite environmentally conscious. At the time it was like ~£1200 for a diamond one and £800 for a synthetic one.
Assuming you mean "late 2018" by "about 5 years ago" (because that's where the graph has a 1.5:1 ratio), that $1200/$800 diamond was probably about 0.2 carat (obviously depends on the other Cs), and would likely be around $1200/$300 today.
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.