Greg pretty much has it covered with the buyer market with the reptile enclosures. It doesn't cost a whole lot to make the enclosures, but the time, labour and space to store them is the killer. This is why the cages seem to be a bit $$$ when people are quoted a price for a unit.
This is a unit I just made that holds 5 drawers of 7gallon and 5 drawers of 3gallon rubbermaids, with heattape ontop as well for other enclosures needed. This paticular unit was relatively cheap to make, approx. $50 for the materials, (without the heat tape) but the labour would run a buyer twice that. This is where the demand drops out.
In all honesty, there is no business for enclosure making, unless you find a way to mass produce them for little $$ and little labour.
Personally, if you wanted to make enclosures or racks to bring to shows, build smaller sized units, with enough room for 4-8 rubbermaids max. That way they are easily moved and apartment friendly. And DO NOT sell units with heat tape. You are liable even if the buyer supplies it.
Hope this helps