Yep, you can pay several hundred dollars easy for a good tone ring. To fit a tone ring on a rim the rim needs to be cut down on a lathe and fitted for that specific ring. It is fairly expensive to have done (I’d guess more than your MM cost new), and you are never sure exactly what you will get until it is all put back together.
Most of the banjos that people spend the money on to put rings in are already top grade banjos. Gibsons out of the 1920’s for instance had a brass hoop fitted to the wood rim. In the 1930’s music evolved to get louder and banjos evolved to follow by adding a tone ring. Many "20s Gibsons are re-cut to replace the hoop with a tone ring.
There is a lot of difference in tone rings and I would recommend playing and listening to several different tone rings in several different banjos before ever buying one.
Banjo Hangout has several sound files where guys have put two different tone rings in the same banjo and recorded it both ways. These are really good to listen to to get an idea of some of the differences in tone you can get from different rings.