Sorry, but could you clarify the dimensions you mean? Are those in feet, inches, etc?

If you mean feet, then $30 for a cabinet that is nine feet by four feet by four feet sounds pretty good.

If it's inches, thirty bucks might be a little dear, but it could depend on the materials used, too.

Tamiya sells some display cases for single car models that retail at $25 for one that is 250mm x 130mm x 110mm, all the way up to $120 for a case for ship models that is 401mm x 101mm x 127mm.  The smaller ones at $25 to $35 aren't priced too badly.  For the largest one, at $120, I'd probably search the flea markets and other places for some made from wood.