When I started riding again several years ago Nationwide quoted me $700 for liability only on a 76 550. Went with Dairyland, it was a little over $300 for the same. When I bought the 750, they wanted to double the price for coverage of both bikes.

I told them that doesn't make sense, I can only ride/wreck one at a time.
Shopped around and found I could get full coverage on both of them from Progressive, with agreed upon values of $2500, and $3000 respectively for around $270 a year.
I could get cheaper, but I like the full coverage, and if I go with claiming it as a classic bike, there are driving restrictions.
It pays to shop around a bit.
Kevin