Cost Effective WAN

I am Eswar working as EX. Systemm in a corporate. The organisation has 9 units in different locations, ranges from 5 kms to 500 kms. My requirement is, HOW TO CONNECT ALL THOSE UNITS with our Corporate office in a VERY COST EFFECTIVE & EFFICIENT WAY? We have to use lotus notes in that communication and pls advise what are means of communications can be done?.