... how the extra gain needed for a (low-output) MC cartridge is realized.
The simplest solution is to increase the gain of the existing high-output/MM amplifier and change the value of the input load resistor. In this case, the marginal cost increase may be as low as the cost of two resistors (per channel) and a switch, so the cost penalty for having an unused (low-output) MC option is relatively minimal.
A better and more costly approach is to add either an extra active gain/amplifier stage or a step-up transformer. The amplifier stage can be as simple and economical as, perhaps, a single transistor and a few passive components or as complex as the designer deems necessary. The cost of a transformer depends upon several factors such as the quality of the material used in the laminations, the winding configuration and how well it is shielded.
On a pure financial basis, it makes no sense to pay for the capability to handle
a (low-output) MC cartridge if you never expect to use one. OTOH, if you prefer the sound quality of a preamp that can be used with both high and low-output cartridges, you need to decide if the added cost is worth it to you.