From USAC eligibility requirements:
Eligibility for Masters National Championships
Masters Road, Track, and Cyclocross National Championships
To be eligible you must be 35 years old or older, hold an annual USA Cycling license, and be a US
Citizen or Permanent Resident. You must ride in your correct age class and may not ride down in age
(with the exception of selected team events on the track that are listed as 30+, 45+, etc.). In addition,
there are a few massed-start events that require a specific numerical category.
In the rule book it does state that for the categories spanning 35-54, it's only Cat 1/2/3 for men, and on the women's side it's 35-44 Cat 1/2/3 only. Men 55+ have no category requirement, and women 45+ have no category requirement.
These requirements could be out of date, I'll update if I hear anything different at tomorrow's announcement.
Haven't heard anything for cost, but USAC nationals is usually in the neighborhood of $90 per event entered. That was in more expensive cities with more difficult courses to control. Hopefully the price is a little lower here!
Tailwinds,
Joey Bacala