By purchasing a used or refurbished iPod touch, and building the RF front-end yourself, you MIGHT be able to set yourself up with an SDR receiver using iSDR for under $100. You can also get an iPhone 3Gs for around $50 with a 2-year contract in the USA - but of course the monthly charges would put the total cost WAY over $100.

Since all the Apple iOS devices are designed primarily to serve other functions (phone, music player, etc), and require a special app and hardware to turn them into MW receivers, I guess they will always fail to fully adhere to the strict definition of an ultralight.

Still, iSDR can provide a remarkably high level of performance in a very portable package. And if you happen to already own a compatible Apple device, then the cost to you is very low indeed.

73,
Charles
NZ0I