How much does it cost to maintain a Trademark?

Trademark registrations are "renewed," whereas patents are "maintained" inasmuch as the USPTO generally requires you to pay maintenance fees for utility patents.

A basic flowchart is available here:

http://www.au-llc.com/trademark-renewal/

Overall, the cost associated with USPTO renewal filing fees is small over time. Applying for and maintaining a portfolio of international trademark applications, e.g., through he Madrid Protocol, can be very expensive, of course, depending on the breadth of protection you are seeking.

Speak with an IP attorney in private to evaluate your proposed trademark(s), where you are looking to expand the business associated therewith, and how you might be able to acquire commensurate protection.