Pushrod engines are more challenging to make them turn high rpm. However Nascar engines are pushrod engines and they turn 9,000 rpm, probably about $100k a copy.
V-Twins typically have 2 valves per cylinder with only one cam (I think). V4s have 2 more cylinders, 12 more valves, and 3 more camshafts. So, yes they are much more expensive to manufacture.