Suggestion would be to post this in a different group. I would like to think you would get a lot more 'hits' than in the WHD group.
For us, we use MPLS for each our pharmacies. Granted they are much further apart, but that would be fairly inexpensive. If you needed to have a much higher bandwidth, we have a Metro-E connection between our main data center and our DR site.
Hope this helps.
A possible cost-effective solution is to use Line-of-sight (LOS) wifi antennas. There are wide selection of antennas which are readily available from local electronics stores.
Keep in mind, you should have a clear line of sight to produce the highest performance, and the performance will be negatively impacted when the weather is bad (wind, rain, hails, etc...) or when a big ship may block the line-of-sight.
If the cost is not an issue and you require a latency-sensitive applications like voice and video, MPLS or Metro-E may be a better solution as edwelly suggested.