Damn, I only pay about $100 a month for full coverage on my 98 Z71 and the 98 Tahoe.
Shop around for a different company. My insurance company will let me add and drop vehicles whenever I want. Plus, if I have a vehicle I only drive once in a while, I can call my agent and put a binder on it for a couple of days when I want to drive it, as long as I give a few days notice.