Just got mine in the mail too and considering the same thing. Wondering if you ever get any flak in pulled over in MA and they see the address on the registration is in ME. I think I may register both of mine in maine this time since they're used only a few times a year (ironically in maine!) and the other times they're stored out of site.
Here in Indiana we pay per year and its by weight. I have one 7,000# trailer, one 12,000# trailer, and one 18,000# trailer.
The 7K cost about $35 per year. The 12K cost about $60 per year, and the 18K cost about $90 per year.
Chris