That must be quite a special ring - it seems that the stone could not cost more than 1000, and this to be generous and assume the best there can be about a I-SI2 below half a carat. Make it anything less than AGS0 cut and eye clena and 6-700 sounds better. Then... what is that setting like? Some could be available for as little as a couple hundred.
Would you consider the thing for half that price
$1800 and 0.5cts diamond and white gold sounds like a designer setting and nice stone inside to me: maybe something like this ring and this stone? Of course, there are many shops where to find something alike - this is just one example.