People really will spend anything on their pets. Take the GoPet Tread Wheel:

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That'll run you $524 for the small size up to $1,260 for the large size. It's pricey and rather unattractive.

A competing product is the more attractive (and still expensive) Cat Wheel, which goes hubless:

The Cat Wheel is made out of MDF and weighs about 53 pounds. The wheel and base both come pre-assembled. If you want to see how the device works, scan through the following vid (no need to watch the whole thing), where Mythbusters' Adam Savage repairs one for his friend:

The Cat Wheel retails for $900 and the shipping is another $300, so you're looking at $1,200 all in.

Entrepreneur Sean Farley figured he could do better and developed his One Fast Cat design.

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Farley's design eschews heavy MDF for molded plastic sections that snap together. The running track is foam. Scan the video below to get a sense of the assembly process and the modular design of the pieces:

Going with lightweight modular components for the structure allowed Farley to get the retail price down to $249—shipping included—and "smart industrial design allows it to ship or retail in a very small box."

In the video below, you can see the product has a bit of flex when in motion, but I don't think that affects the function:

When Farley put the project up on Kickstarter last year, he was seeking $10,000. He wound up with $344,911 instead, and today One Fast Cat is a going concern.

So, those of you ID'ers looking for a design niche with a ready market and products with room for improvement, we'd say pet products are worth looking into.