How much does it cost to maintain a Trademark?

I guess the answer you are looking for and the only way to answer such a broad question is to explain that virtually every country now registers a mark for a ten year period. The renewal fee for just the trademark office at the end of those periods varies from probably around US$200 to US$2,000 depending upon the country or if a whole region is covered, like the European Community, and how many classes of goods/services are covered. Attorneys fees, if an attorney is retained, usually costs less than US$1,000 and probably more like a few hundred dollars but there's always exceptions. Like another answerer said, in the scope of things for a large company, paying that much every 10 years would not seem like a lot. If you multiply that by a dozen or several dozen countries, I guess you just need to work that into the IP budget of the company.
The U.S. and possibly some other countries require a maintenance filing between the 5th-6th year after the registration to show that the mark is still being used and that costs $200 per class plus attorney's fees.
Ideally, you are working with an IP attorney and that IP attorney should be able to assess your whole portfolio of marks and provide you with an estimate of how much to budget for the maintenance of the marks. Also, keep in mind that these costs are only to maintain the registrations in their respective trademark offices. To really maintain marks, including and especially registered marks, there should be enforcement actions against similar marks in your industry and that can require an even larger budget. Because if a number of similar marks become adopted by companies in your field, you may effectively lose the mark on that basis even if not yet expired in the trademark office.