If the casting costs $200 I don't think there's any way to make profit on 100 sets. I'm a machinist and have designed and built many fixtures to hold casting while they are being machined. Making a reliable fixture that puts out consistent product takes time and money. One fixture would have to be made for each manifold then a machine programmer would have to develop (write) a program to run the CNC machine. Then the CNC machine would have to be "set up" for one run of 100 then "torn down" and set up again for another run of the other 100 manifold. I would estimate a cost of $3000.00 (rough estimate) to make/test the fixtures and to program the machine. Then to set up each run would be another cost, after that all you have to do is pay for the machining itself.
You'd probably have to make a run of at least 500-1000 (if not more) sets to be able to drive the cost down to an acceptable level. Fixtures and programming are a one time cost. Each time you have to "set up" for a run is a recurring cost. The cost of the machining itself should be consistent.
Most of the cost (up front) will be in designing and making the fixtures, testing the fixtures, writing the program, testing the program.
PS - We didn't mention any testing to ensure reliability/durability which takes time and money.
Treblig