Why do they cost so much? Tuning of the airflow is why.
With NA engines you need to get the shape right in order to feed the cylinders properly, an incorrect design will shroud one or more of the cylinders and result in starvation in those cylinders (an alleged problem with LET engines running over 300bhp with a stock inlet manifold).
Also if the capacity of the plenum area is wrong then that can cause problems, too little room and you'll create a shrouding effect anyway, too much and the furthest cylinder is starved.
Then the runners are important as these are pulse tuned to provide the characteristics for the engine, hiding flat spots usually. Long runners normally mean better torque, shorter for more power.
Is it possible to have a custom one made? Absolutely, as long as you don't want it to be pretty then you can put on development ones and sit on the rolling road for the remap to see what works, but then this does normally take a while to do unless you have a bit of room to play and sliding runners. Once you have the setup you want you can have the proper one made, or weld the development one up properly.