The only real cost difference is the panel itself and the feed wires are of a larger gauge (and cost more).
They are the same type (depending on the brand of panel you buy). Plus, the cost of the breakers is the same.
If I remember correctly, a subpanel can be up to half the amperage of the main panel (for example, up to 100 amps with a 200 amp main panel). Either way, you still have to have a shutoff breaker for the panel. I have a 100 amp sub panel with 12 slots in my garage and the 100 amp breaker is in the main panel, not on the subpanel (it doesn't even have a slot for a main breaker). The breaker for the subpanel is located in the main panel. I can see a smaller box costing less, but a subpanel typically doesn't have a master breaker in the box regardless of size.
