With regard to no difference between a 2 color and a 20 color print, the key is that both prints have the same amount of ink coverage. Prints are printed nuetral cotton and then are covered 100% with the design (most of the time - some prints might leave neutral areas). Once the die board has been made, there is usually no additional cost in adding 2 or 20 different inks (with exception of metalic inks I presume, but the price usually reflects that). So they're using the same amount of ink for 2 and 20 color designs.
The other cost addition is the die board as it's more complicated/expensive to make a 20 color die board than it is a 2 color. However, when you spread the cost of the die board that can be used hundreds of times over the thousands of yards that will be printed, the cost is negligble.
I was a cost accounting for a printing company for 20+ years. We printed on paper instead of fabric, but the accounting will be the same :)