Because the head is designed for economy & emissions- which is actually why it has 16 valves. The cosworth head is obviously a peformance head as std, and takes a hell of a lot of beating. The cvh hemi head as used on the RST, has HUGE valves as std, and mainly needs work round the valve seats - and shaping of ports. The zetec head has to be totally reworked to convert it from emissions to peformance. It takes twice as long as a cossie head to port - which should give u some idea on price!