Best solution to get 3.3V from 5V-20V @ 250mA
There are number of companies that offer drop in switching regulators that might meet your needs.  I'd look at some of the electronics distributors out there with good parametric search capabilies like Digikey, Newark, or Mouser that might have something already designed that will work for you.

Although it's a little more expensive than your $3 cost target.  Look at this drop-in switching regulator.

It handles the output current and input voltage range you've specified.  It's $5.10 in quantities of 100.  Take into consideration the time saving cost benefit of not having to design one yourself.