OK, I read the article and watched the TV report. No mention of a $6000 installation figure per sign anywhere. The OP needs to come up with a specific, factual source. To me it sounds like complete made-up BS. Just more emotional porn propaganda so political junkies can get an anger-gasm out of something. Somebody needs to cite a specific, factual source for that figure. The signs look pretty cheap and temporary in nature. Coroplast on wood posts for the most part.
The only problem I have with the SB1 signs is where they're being installed: close to the road within the highway right of way. Only actual traffic control signs are allowed within the highway ROW, and those have pretty strict rules to follow. Signs not specifically designed for controlling traffic are to be installed outside the highway ROW and utility easement. The one guy in the video fighting the fuel tax hike has a point with his complaints against those signs.
As for the fuel tax increase itself, the cost of living and doing business in California has become so insanely expensive that it only figures the price gouging would eventually affect the cost of building and maintaining roads. The labor gets more expensive. And the materials aren't getting any cheaper either. A few months ago Oklahoma had its first fuel tax increase in 25 years (a 3¢ hike on gas, 6¢ on diesel). The CA fuel tax hike is quite a bit higher. But I think the folks in California need to look at some other things when complaining about price gouging in that state.
Side tangent: I'm not a big fan of California's highway signs. Many of them are really ugly. They have some old vintage button copy signs, but most of those are in bad shape. I can't think of any other state that does patch jobs on big green signs like Caltrans. I think Texas does a much better job (mostly). Here in Oklahoma things are hit and miss.