I think by the time you bought the motor for 10-12 dollars, found something to house the motor & batteries in or wired a larger battery to several and fashioned a wing and shaft, you could probably buy the Mojos as cheaply as you made them. I know from all my projects that it's more satisfying building your own as I'm the world's most fanatic tinkerer. Maybe thats why you want to build your own. I usually do that on expensive items like the Vortex, the Goose Flagger, the Butts Up pumper, Weasel Ball Mallard, original Mojo, etc.
About 8 years ago, our local Walmart obviously overbought on the Wing Thangs. End of season they put them on sale but were disappearing from the shelf kinda slowly. When they got down to $12.99, I went to the store manager and offered him $8 each for the lot. He accepted and I got 21 Thangs for 168 bucks. Kept 6 for me, sold the rest on eBay and they averaged a little over $17.50 for the 15 that I sold. Keep an eye on Walmart.