Hi,
The answer to this question requires knowing what data rate your sending, is it continuous, is it secure data etc.
You can connect 2 offices to backup or sync small amounts of data by the easiest method of a land line at 56K. If you wish to send more data then go DSL or ISDN.
The cheapest method is if you have internet service at both offices you could connect that way (again for small amounts of data), mind you data security becomes an issue then.
If you don't have a landline provider then you can always look into UHF packet transmission systems (again depends on data rate and budget).
If you have an unlimited budget and can lay down 50K+ of fiber (mega bucks) then go that route.
Regards