Just my 2p worth... I'd say you have a lot of designing still to do. My guess is that you will replace some of that extrusion with plate and you'll probably add more (machined) plate into the design. Any guess at a machining cost at this stage would be unrealistic I think. You might also like to consider whether your design will require machining on both sides of any plate as that will increase the cost due to the set up time needed to machine one side, flip the plate and align it accurately for machining on the other side. If you have a look at my design I've specifically made it so that it can all be machined from one side with the exception of some counterbores that I'll do myself. Lets put it this way, I spent a lot more on parts than I did / will on machining costs.