Just expanding a bit on Shanky's response...
The options available to you, and the price associated with each, are extremely dependent on where you are. In many countries, the Government runs the telephone system and as soon as you need a link even across the street they take advantage of their monopoly position. Even in the US (which is implied by your use of $ and miles as metrics), there are huge variations as you move around the country or in and out of major cities.
At 50 miles, you are out of range of typical LAN oriented solutions, and will be looking at either a MAN or WAN based solution.
A major consideration is do you really need 100 Mbps? Your WAN based solutions are based on telco signalling, and you would have many more options available to you if you were willing to settle for a 45 Mbps (T3 or more properly a DS3) connection.
Your request for 100 Mbps implies to me you are looking at connecting two LANs, and thinking in terms of bridging rather than routing. Remember that you only need to carry the traffic which must go between the sites, and a routed link does not carry all the broadcasts and other riff-raff common on a LAN.
High data rate wireless solutions, whether licensed or unlicensed, are limited to line of sight. This usually limits you to about 25 miles per hop because of the curvature of the earth, but wide variation occur in real life due to local geography and what buildings/mountains/obstacles are between your two sites. Unlicensed wireless is also subject to interference from other uses of the spectrum while licensed wireless is subject to unavailability of licenses.
Infrared is another form of wireless, but again you need a clear line of sight. As Shanky noted, you may be able to get a clear line of sight by putting together multiple hops. You also have to be more concerned with rain, snow and fog.
Finally, when engineering your solution, remember to consider not only the bandwidth required but also the delay allowable. This includes not only the propagation delay due to the finite speed of light, but also serialization delay (e.g. 1000 bytes at 10 Mbps requires 0.8 ms), processing delay (can be significant if doing compression or encryption), and queueing delays (the real killer on high utilization links).
For what it is worth, your quote of $13000/month is not that exhorbitant for the three T3's required to meet your demand for 100 Mbps. Changing nothing else, I suspect you could get the monthly bill well below $5000 if you drop your demand from 100 Mbps to 43 Mbps.
At the risk of sounding self-serving...for what you'll be paying per month to connect your two sites, a few thousand dollars invested in a competent consultant who could help you sort out your real requirements and formulate some alternative solutions would be an excellent investment. Even if he/she saved you only $500/month, you'd get your money back in less than a year and the rest is profit you get to keep.
Good luck and have fun!
Vincent C Jones
www.networkingunlimited.com
