Are there any costs associated with specimen submission?

Specimen submission is priced depending on the form you are submitting it with:

• When you register a new trademark for a brand which was used in US commerce before you filed your application, you will submit the specimen of use with your applicaion at no additional cost.
• If you apply for a new trademark for a brand that isn't actively selling in the US yet, you will naturally have no specimens to provide. The USPTO will request them from you once the initial examination of your application is done, a few months to a year after filling. In this case, you will be required to file a Statement of Use, which will also include your specimens. Filling an SoU is priced at $100 per class of goods and services.
• If you have a registered trademark in the US, you'll have to file a Declaration of Use between 5th and 6th year of its validity to prove your brand is still used in the US commerce, which will also include providing specimens as evidence. DoU is priced at $225 per class of goods and services. You will have to file a DoU every ten years after that.
• And last but not least, after ten years of validity, when you want to renew your US trademark, you are also required to provide specimens. US trademark renewals are priced at $525 per class. Renewals also occure every ten years.

This means that to keep your US trademark registration alive, you will have to provide specimens every five years, either with a Declaration of Use or a renewal request.

What's important to reiterate is that the specimen submission itself is not subject to a price, meaning it doesn't matter how many files you submit. The fees apply to the submission of the related forms and the final cost depends on the number of your classes.

Please note that the mentioned prices reflect USPTO fees only. If you have an attorney help you with specimen preparation/submission, their services will be subject to additional costs.