There are full differentials available for about $100 or so. It wouldn't break the bank, but probably is only necessary for dirt and gravel.
Many of the one-speed upright trikes have coaster brakes. Perhaps also 3-speed. The designs I've seen have an idler shaft in front of the axle. A "poor man's differential" would simply run two freewheels on the axle stubs, but one would loose the coaster brakes.
Chromoly might add a little to the cost, but wouldn't be more than a couple hundred at most. It is probably more in marketing. Thinwall chromoly does occasionally dent, but a well built frame should last a lifetime.