I had heard that a long time ago, it only cost $10 to replace any strap or shock ring, then rubber prices went through the roof.

I think the people at Soloflex have tested other alternatives and found them to be inferior to their straps, or even more expensive to get manufactured.
The shock rings they have available had to be produced in bulk to the specific size for the Soloflex machine. I think they have a warehouse full. The weightstraps and shock rings they have I think are of the same quality that goes into aircraft parts. (Jerry the founder used to be a pilot). I also think that when a replacement strap of any weight was available from Soloflex for $10, their cost was around $2. So far as I know the price for them to get straps made is much higher than that and their margins are much smaller.
All of Soloflex's costs have gone through the roof. When they started out, the cost of weightstraps, shock rings, steel, rubber, shipping, and advertising were very affordable. All of those costs have gone through the roof, and peoples incomes have not matched those increased costs. Back then it cost as much to advertise it as it did to get it built to get the sale.
If there was no Soloflex in existence and they were just starting out from scratch today, they would not have been able to manufacture in the USA. They would have been forced to build it overseas and make it much lighter and it would not match the quality we currently have. I know this because I had my own designs in the past of exercise equipment I wanted to get made up, manufactured and sold. Everyone told me only build the prototypes here. If I did not get it manufactured overseas it would be way too expensive for anyone to buy and there would be no chance of it being a successful venture. They also said that 30 to 40 years ago it would have been possible to do so in North America successfully, but not now.