I really wish there were cheaper Tx capable SDRs, as they look like a lot of fun! The cheapest ones I've seen at the moment seem to be around $200 hackrfs from aliexpress.
Heck you can pick up a Mobilinkd[1] for ~$60 if you just want to stick with KISS/AX.25 over Bluetooth. They're really awesome little devices for what they do.
[1] https://www.adafruit.com/products/3076 [2] http://www.ebay.com/itm/162086931088
The chip itself covers 119–1050 MHz, but the modules I have, have filters on to allow only narrower ranges.