We went through the same issue. With a few exceptions, rates are in the $200 to $400/mo range. The only exceptions I found were in Pt Robert, Olympia, Anacortes, and, of course, Bellingham. I am still not sure how much they will be since some of them may treat the truck and trailer as two separate entities.
I was culture shock for us. In the Cleveland area, launch ramps and rig storage launching into Lake Erie are all free. Inside heated and AC storage is in the $200/mo range.
We are launching in Bellingham. Storage is free and mostly (one guy on this list temporarily lost a trailer, I am told) secure, at least we hope so.
I have quite a substantial list of people to call - I was planning to do that till I found out about Bellingham - which I would be happy to send you if you want to go that route.
John