There are several key reasons that MPUs cost so much less.
First, the MPU does not have further processing capacity. That is, an MPU -- which we should really call an MSU for "mobile slaughter unit" -- only takes the animal to a pair of clean, half-carcasses. The MSU has to have a relationship with a further processing facility -- which is a fixed facility -- that will take those half-carcasses and turn them into labeled packages of meat for sale.
A fixed facility would never just do slaughter; they always do further processing also. Hence the additional costs.
Second, a MSU is mobile: no land cost.
Third, a MSU is smaller: lower construction cost.
Fourth, a MSU needs far less equipment: lower equipment cost. If you build a fixed facility, you are unlikely to build it as small as an MSU, with that little capacity. So you are going to need more space, more equipment, etc.
There are other reasons as well, but those are some of the big ones.
The Niche Meat Processor Assistance Network has quite a bit of information online about mobile units: http://www.extension.org/pages/Mobile_Slaughter/Processing_Units
Browse related Faqs by tag:
small meat processors