Carl,
I have worked for wireless companies for over 20 years. I have maintained the equipment in these location for 8 of those years. First, the video of the tower burning and dropping is an extremely rare case...so I wouldn't even consider it. These things are engineered based on type, height, loading, wind and soil conditions of your location. They are heavily regulated.
Next is the non-ionizing radiation from the tower. Basically, frequencies are reused every seventh cell site, this means that as they infill (add more towers), the output power goes down (as in your case). Also, modulation techniques have changed greatly since cell phones came out. The newest is Wideband CDMA technique (hidden in the noise floor and tightly regulated power consumption) are very low powered and broad frequency. Also, the antennas emit a direction (sector) pattern and would have most of its power outward and not below...so your neighbors down the street are more affected than you...however, with the distance, its a non issue for them either. So no issue there...I have worked with 100 of guys for years in this stuff and NOBODY among us has had cancer related to non-ionizing radiation. I would be far more concerned at the exposure to carwash chemicals that the cell tower.
Finally, $750 sounds low. I get $500 a month for 4'x4' for a bank ATM machine. But it depends on the location, access, etc. Getting free phones use to be common, but now most carriers have sold off the towers and just rent space on and below them. The parcel size required depends on the type of tower they erect (monopole, self-standing or guyed).
If your not using the land, I'd haggle a bit and take it.
Big