Yes, there is indeed one cost to keep in mind if you own a US trademark.
During 5th and 6th year after registration, you have to submit a Declaration of Use declaring the trademark is still used in commerce, meaning the products/services labelled with the trademark can be purchased/ordered by US customers. The DoU has to be submitted together with examples of this usage (a so-called "specimen"). You have to provide a specimen for each of your classes. If you cannot provide it, you risk cancellation of the trademark in the concrete class or as a whole.
As for the price, the Declaration of Use is priced at $225 per class of goods and services. Please note this is just the government fee charged by the USPTO. If you are using the services of an attorney or a law firm to help you file the DoU and/or put together the specimen, their fees will also apply.