I voted, but it probably skewed the poll as I got engaged in 1977. My .50 ct oval was $456.00. It was a real stretch for my husband and took every penny he'd saved. The stone now lives with a small pair of diamond earrings in a 3 stone necklace. Recycled!
My engagement ring was bought in 1982. It is a .38 ct RB in a YG tiffany style setting, and its H color VS2 clarity. It cost $500 which was a decent amount of money back then. You can google what prices of everything were in the late 1970's/early 80's and see that $$ were just worth a lot more.
I bought an anniversary ring which is a platinum solitaire tiffany style in 2008 on Blue Nile. The diamond cost $1700--it is a .66 F SI1 AGS0 ideal cut. the ring itself cost $700. You could still get a ring like that for the price at Blue Nile and it would be about $300 cheaper if you went with white or yellow gold. That is the online price though. I compared prices at upscale brick and mortar stores and the exact same ring was $3500- that is $1000 more.
Many girls want a ring that is 'at least a carat'. I bought earrings that were .91 each (looks like a carat) H VS2 AGS0 ideal cut in 2009. This is a good quality of diamond for an engagement ring in the US. Each diamond was $5000. So had I put one of them in a plain platinum Tiffany style, I would have had an e-ring for $5700. Of course if I wanted a fancier style, which some girls want, I would have paid more--or got a smaller stone.
If you find a good pawn shop, prices get quite different. I had a friend bought an e-ring recently at a pawn shop, 1.25ct diamond EGL-USA graded g VS (it had the cert) in a white gold ring with baguettes at the side (I don't know the total carat weight)--and she paid $5,000. (Or rather, her husband did). There is no way anyone could match this deal even online, much less in a B & M store.
If you are willing to go with a diamond that is not a round brilliant, you can get even better deals. I just bought a marquise diamond, 1.20 carats, I SI, with a cert at the same pawnshop, with 28 diamond sidestones totalling over 78 points for $1500. Marquise diamonds right now cost very much less than RBs because they are not in style--but they are beautiful.
So there are all kinds of choices in different venues.
Blue Nile IS a good place to get an idea of ring prices on the internet. there is a place where they tell you what was recently bought and show the rings. The rings range from usually $500 up to $70,000, sometimes $100,000. Most purchases are clustered in the $1500-$6000 range. This means, most people who buy there range from about 1/2 to 1 carat diamond, in different qualities and the rings themselves cost different prices depending on more or less elaborate. Remember though that Blue Nile has an affluent customer base. But go there yourself and look.
Hope this was helpful