I would look in to a in-house wireless solution, two point-to-point antenna's on the roof of each office. That way, you will only need to purchase the equipment - which will run you several thousand dollars. But besides probably having to pay the city a little bit for a license/permit (you would be using airspace, and the city CAN revoke the permit if your setup interferes with anything - in theory) you won't have a monthly cost.
Speeds run from 54 Mbps (802.11g with extenders) to several 100 Mbps.
See here for some info: »www.gnswireless.com/wireless_bridge_kits.htm
You are very likely to see complete return of investment vs. paying AT&T for a leased line in the first year.
Only problem might be line of sight.... a higher building in between means this is not an option.--
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