Wow $300? I pay $99 & $120 for my 2 cars. What make/model year is your car If I may ask? Also if you are with PEMCO (many in WA are...) you will need to switch to a new car insurance provider who operates in CA (like Geico, State Farm etc...) and get a refund from WA insurance for the rest of the unused term.

Never let your registration expire. If you ever get into an accident (knock on the wood it doesn't happen), your insurance will NOT cover a dime and you will be in severe trouble. Also, some insurances do not cover if you register your car in a different state, though they cover accidents in different state (so you may have to get a different insurance provider)

I used to live in CA they are so much annoying with paperwork, some DMVs in CA ask for proof of current out of state registration, if you don't they make you file additional paperwork & fees. IMHO, not worth the hassle. I know WA tabs cost $100 - $150 bucks, it sucks when you don't use them.