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 April 2019: Note that the bankruptcy of Radio Shack means a lot of formerly common parts can no longer be purchased with cash at a corner store.This can seriously slow down construction if you have to wait for some 99 cent part to be shipped after running short or accidently destroying one. If you have to order small parts online, be sure to order spares, and use anonymous prepaid debit cards to pay for them.
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.
DC Direct Action News 
please in your diagram for the mixer,the center tap is found on the coil that provided an rf ground (this was in the 2013 research) but subsequent ones,there is no center tap there,please can you tell me why and why not the need for a center tap there?. I have spent about two years building your circuits,but i could NEVER get my IF out on the frequency counter,whiles i can tune it on a digital radio. PLEASE I NEED YOUR HELP,my email is mawulolo95@gmail.com
All of the RF mixers I built used the capacitors (not the coil) for the output side RF center tap. On the high frequency input, the capacitor provide the tap for VHF crystal oscillator frequency input and the center tap on the coil (which must be accurate) proovides a ground to close that circuit.
Do keep in mind that if the frequency in question does not show up on a wavemeter, spectrum analyzer, frequency counter, etc but can be tuned on the receiver, the dominent energy is on some other frequency. The desired frequency will always be there but may be too weak to pick out and make use of though your receiver still finds it. This mixer is “picky” about how much energy from each side goes into it etc, you will just have to experiment with what gives the best output.
Note that it has been YEARS since I have worked with this, due to the collapse of FM radio listership other than those who just sit on a single channel and never “spin the dial” I think it was the high side frequency that needs to be considerable stronger to get a clean output but like I said it’s been years since I’ve worked with it.