What is the cost of one simple gold ring with one carat diamond?

There is no fixed price for such an object.

If you're shopping for the lowest price, you are best advised to shop after-market options, such as consignments, or personal sales. Educate yourself first, to be confident that the price you're paying will get you the best value for your money.

You can buy a round brilliant diamond today on Blue Nile and spend from ~US$3400 to ~US$4200, then pay for the mounting for the stone.