You can purchase a 10 carat loose diamond today at Blue Nile and spend between US$234,000 and US$1,187,000. Then you'd have to pay for the ring setting.
Since diamonds are valued according to more than the carat weight, including colour, cut and clarity, you can shop at a local jeweler who can show you stones of this category, or refer you to someone...
You can buy a round brilliant stone weighing about 10 carats today at Blue Nile and spend from US$238,000 to US$1,272,000, depending on the clarity and colour of the stone you want to buy.
The money you pay for a diamond depends on its cut, clarity, carat weight and colour. You can spend several hundred dollars or several million dollars for a diamond.