It depends on the boat and where you are sailing. The speed differences between a fixed prop and a folding prop, in moderate winds can be very small but in light air and in marginal surfing conditions the speed gains can be huge. So sailing on a lake, or in an area like the Chesapeake or Long Island Sound, folding props are really helpful.
Folding props have their greatest impact on compartively light weight boats, boats with small sail area to displacement, and boats with fin keels and spade rudders where the prop is hanging out in the free flowing water stream. For boats in those categories going to a folding prop can easily be worth it. The question gets dicier with a heavy cruising boat, where the prop is a proportionately small amount of the boat''s over all drag.