Wayne,
When you compare the bump in revenue at these pubic venues to that of the hundreds of millions spend and the years upon years of course construction endured by the members at places such as LACC, Brookline, Winged Foot, Oak Hill, Southern Hills, Baltusrol, Oakland Hills, etc... It would still seem the private clubs are willing/able to endure quite a bit more to continue to host majors.
The public courses don't have the luxury of spending money on speculation, nor always jumping through the hoops of hosting lesser events to get to the same status. Unless it's already been announced that your course is going to host a US Open, you need deep pockets to chase that potential revenue stream.