I've recently(well in the last year) have fulfilled all requirements for my 25 ton OUPV.
This what you'll have to do to:
Pass the exam...highly recommend taking a course
They test you on site. $1000 depending on where you live I took it in Sausalito and decided to stay at a hotel rather than commute.
Pass a stringent Physical , 9 pages to be exact
Aquire a CPR first aid certificate $85
Submitt 3 letters of recommendation
Aquire a TWIC (Transportation Worker Identification Card) ID card $135 basically your background check from Homeland Security
Pass a Drug test (note the drug test must be within the last 6 months)
Submit your sea time documentation ( note you can use your own vessel for this but I recommend aquiring as much sea time as possible on other boats. You will need CF numbers boat owners signature and contact info. 360 life time 90 of the 360 in the last 3 years. "Sea days" is minimum 4-6 hours underway.) if using your own vessel for part of your sea time you will need to show proof of ownership of said vessel during the time of its use ie insurance documents, bill of sale etc...
You have 1 year once you have passed the exam to submit your application to the coastguard $148
I would also recommend submitting your application in person at the nearest coastguard regional center.
Your basically joining the merchant marines which allows for you to apply for employment on freighters etc...
Overall it cost around $3000
That last part...to ship out on anything bigger than a crewboat, you will need STCW which costs a good bit. Your 100-ton license doesn't get you anywhere on anything bigger, even Ordinary Seaman. You need the STCW classes/drills, it's an international treaty requirement.
Back in 1980 (!) my cost was probably about $250, of which maybe $100 for doctor, 50 drug test, 40 for prep books, 50 for application fee, thereabouts. Yeah 90% is tough for navrules (lights for submarines and seaplanes and dredges) but doable. The exam questions are posted on-line if you look. You could probably double or triple my costs for home-study license today. And you have to get the TWIC.
Simpler times back then, but I doubt the navrules exam has changed much.