1) The spendy ones have nice handles and automatic pressure release. The cheapo ones have to be adjusted with a screwdriver, and you manually release pressure when you go from a higher to a lower pressure.
2) Is it worth it to you? I bought the cheapo ones, and I am perfectly happy with them.
3) No. Doesn't matter if you spend 10 cents or 10,000 dollars, the beer will pour the same, (as long as you have either set at the same pressure).