The cheapest way to get a link that is going to have some sustainability to it is going to be using ubiquity products. They are considered "Carrier Grade" and have a proprietary TDMA protocol to mitigate interference from other AP's on the same channel. Since you're only going to need around 3-6mbit at the most, I'd recommend using a Ubiquity Nanostation M5 at both ends. Light, comes with POE injector and has a range of about 11 miles PTP between 2 of them at reliable MCS-12 - 15. They are also MIMO devices, which helps if there is some unusual RF activity in the range selected. All devices can be either a AP, AP WDS, or Client / Client WDS. Configuration is straight forward, and they are high power transmitters.
Recommended configuration flying blind:
5mhz channel spacing, Enable AirMax, enable encryption, set to AP WDS / Client WDS, configure in the upper most channel (use Airview spectrum analyzer thats built in if you're up to it) and
Set transmit power to 23 DBM, aim em approximately straight at each other, and go for it.
These things have a fairly wide azimuth antenna, but at the distance specified, it will not be an issue. The devices range from 80 - 120 depending on where you get them.
Another cheaper option from the same company is the BulletM5HP -- Same concept, just NOT MIMO, and attaches to any antenna of your choosing.
Be sure when buying these things that you upgrade to 5.2, most all of them come shipped with it, but just in case.
They are also low latency, especially compared to say 2 linksys WAP11's. Typical ping times on mine is about 0.3 MS per link at 20mhz width, at 3 miles a pop, and 1.7 with a 31 mile link running 40mhz channel width. The higher the channel width the more latency and more interference. Stay away from the highly populated 2.4 b/g band for best results with a PTP link that is NOT going on a tower. Also, be considerate of others... there is likely a tower some where near by that is in the line of transmission of your equipment.. try and use Airview to find a dead channel, and then only use the required amount of transmit power to get -63 signal.. This is being nice since its the ISM band and its anyones game, but why screw with some one if you dont have to? Also when aiming be advised that on most antennas down tilt / uptilt is much more of a game changer than left / right, since the beamwidth is usually tighter.
Hope this is of use.