Kosher meals cost more because they have to be prepared in a special way. Separate from non-kosher meals, no mixing between meat and dairy, and supervised by a Halachic Authority, there has to be a hechsher. They need to pay the person who is supervising it and inspecting it. The leaves of the salads need to be washed in detail. The meat is slaughtered kosher and prepared in a kosher way, which is automatically more expensive than non-kosher meal.
In short, yes kosher meals will be more expensive.
It's great that non-Jews are interested in kosher meals. But in prisons, it seems more like they want to kosher meals not out of a spiritual reason but just because kosher meals are more palatable than non kosher meals. So I don't think non-Jews should have access to expensive kosher meals that the taxpayers pay for just because it's "better" tasting. If they were doing it for spiritual reasons then I would fully support non-Jewish inmates getting kosher meals. If anything, the prisons should serve better non-kosher meals to non-Jewish inmates.