It’s all the same $145 cost to test for every rating. Most of the exam questions come from the same pool. None of them will be very difficult. You only need to get 70% right. You can retest for any modules that you flunk. It would be silly not to take them all.
I don’t know if higher ratings cover lower ratings, maybe, maybe not. It does not matter what the rules are, if you don’t get everything you can, you’ll undoubtedly encounter HR girls with a checklist that don’t know the rules, and just throw your CV in the trash because you don’t have the particular rating that is on her checklist.
If you need to worry about whether the tests cost $145 or $500, you are getting into the wrong business. Most of us are spending several thousand dollars a year on USCG approved STCW courses.