There was a related thread a few days ago that identified
this source. Prices are much lower than the Amazon price, BUT shipping increases the effective cost quite a bit. The person who identified the source said he had purchased belts from this supplier and vouched for the quality.
I ordered a replacement belt from PSI shortly after I bought my Turncrafter about 5 years ago. At the time, they sold in sets of two for around $20. Both Amazon and VBelt Supply are much less, even with the high shipping.
No surprise - any time you purchase renewal parts from the original manufacturer/distributor, you have to expect to pay a premium price. Just look at what parts for you vehicle cost at the original dealer compared with NAPA! And that's not criticism - it costs money to stock miscellaneous parts, and that cost has to be carried forward as a premium price in return for the convenience of purchasing them at the dealer.
I had to replace the belt on my lathe after five years - in my case, the symptom was that the belt got pretty noisy.