Gun rights advocates fear background check legislation not because of background checks, but because of the guy who inevitably shows up and says "Why do you need an AR15 or a magazine that holds more than 10 rounds?" "Assault weapons" kill fewer people each year than ladders and cops, and handguns are responsible for 90% of gun crime, yet "common sense gun control" advocates go first and foremost after the scary black rifle. To gun rights advocates, this destroys the legitimacy of the pro universal background check effort.
Want universal background checks? You'll need moderate, left-of-center gun owners like me to support it. Pass a law mandating universal background checks that are both accessible and inexpensive, and also codify that I have a right to my rifle and whatever magazine size I want, including criminal penalties for any law enforcement officer that confiscates my rifle without a warrant, and I'll call my Congressman to support it. Seek to push it further than that, people like me will fight it tooth and nail, and the status quo will remain. That's about as honestly as I can put it.
We might all hate him but $50 million is nothing to laugh about. Money runs our government and the top dollar buys elections. Not to mention that 50 million is just the tip of the iceberg. Bloomberg could easily throw in an additional 100-200 million per year if he sees fit to do so.