The link below is to a pdf file containing the conclusions and result of several years worth of research and development by DC’s WSQT Radio into building FM broadcast radio transmitters using parts from Radio Shack, the dumpster, hamfests, an electronics parts store (found in most larger cities) and Ebay (for VHF/UHF output power transistors that can be found nowhere else.) A transmitter making nearly 100 watts can be built for no more than a couple hundred dollars, far less if you are lucky at the dumpster and on Ebay!
UPDATE June 2013: The pdf file below is updated to reflect another year of research and the construction of a new transmitter producing up to 90 watts at 12.5V with a cleaner signal built from somewhat more common parts. Both MRF648 and MRF247 output transistors have been tested. Slightly more output in this circuit from the MRF648 but the output circuit could probably be matched for a little higher output. As shown expect 80W from batteries with either device at 12.3-12.5V, 100W should be possible on a regulated 13.8V supply on AC power.
Older documents from 2012-this transmitter wasa deployed from Summer 2012 to Spring 2013
As of October 6, 2012, the then-used WSQT transmitter would make 85 watts at 12.5 volts and 100 watts if hooked to a 14 volt power supply.
Note: there are a couple errors in the older schematic that were of course not on the transmitter, stemming from late-night transcription from the circuit board to paper.