Most cost effective is a relative term... In general, there are three key parameters: cost, security and performance; and you only get to choose two out of three. For example, if your routers do not support IPSec, upgrading to a feature set which does is a significant chunk of change. But you can communicate without using IPSec, you just won't have the same degree of privacy or access control.
You do not specify what your requirements are for performance (speed, availability, quality of service, delay, ease of use, etc) or for security, or give a hint as to your application. The absolute cheapest way to connect two sites through the public internet is to live with dynamically assigned IP addresses, exchange IP addresses over the telephone, use NAT to provide access to/from the boxes behind the routers, and let your boxes talk to one another. This might even be adequate if the requirement is simple ssh access to a UNIX box to allow remote maintenance.
I would suggest sitting down with a competent consultant for an hour or two to discuss your real needs and determine what you actually require. Then come back to this forum to validate the answer. But make sure you are working with a competent consultant and not a sales droid with a product to sell. If the "consultant" starts the conversation with their presentation of the perfect solution, guard your wallet and look for someone else. Be forewarned that if there is no charge for the conversation, it is a sales call and not a consultation. There are a wide range of solutions available and what is appropriate for someone else may or may not be what is best (or even useful) for you.
Good luck and have fun!
Vincent C Jones
www.networkingunlimited.com
