I can offer my 2/100th of your prefererred currency...
I've been running a K2 CNC KG3925 for a while now. This is a moderate hobbyist machine, with a 39x25" work area. It's popular with guitar builders, and I've found it'll happily munch through MDF for loudspeaker construction.
The main thing I've learnt is that CNC is not a panacea; to go from an idea or box drawing, to something you can cut takes time, and sometimes the creation of scrap!
You have to remember that parts can only be cut from one side. Cutting a rebate for a driver flange on one face, and rebates to help with box construction on the other, requires flipping a piece over and realigning it perfectly for the second cut - this is very timeconsuming. Basically, it's much easier to design your parts so that they require only cuts from one side.
Also, unless you have a vacuum holddown system (and I'd suspect that few hobbyists do) then cut parts need tabs to hold them in place, which have to be manually cleaned up afterwards.
In short, cutting a set of pieces from an MDF sheet on my machine may take anything from a few minutes to a few tens of minutes, but the total setup time of loading a sheet, cutting, removing parts, cleanup etc. usually triples (at least) the production time.
With a more expensive "small business" machine (4x8' cutting area + much faster cutting speeds) I suspect the overhead time per set of parts would be much less. I did look at producing parts for a sub on my machine with a designer a while back. The problem was that the sheer length of time it would take to cut all the parts (several load, cut, unload cycles per box) meant that it would've been impossible to sell them for a sensible price and make any money at all.
As a real world example - there's a UK based DIY speaker place I know of, and they will sell you kits of parts (drivers, crossovers etc.), the same plus MDF panels, or even completed boxes. The cost of a set of MDF panels from them range from approx 170USD (for a sub) to 500USD (for a big floorstander).
When you factor in the cost of the raw MDF sheets, transportation of materials, machine running costs, dust extraction, operator time etc., I'd consider these prices to be fairly reasonable.
It is possible to make money with CNC - there's plenty of guys doing it, but I suspect you've either got to be producing lots of simple parts (i.e. volume cutting parts to a design you know and trust for your machine) or producing high value items (like using the CNC to reduce the workload when making a custom guitar).
BTW - jinjuku's comment about using a flush trim router bit is a very good one. Even with CNC cut parts, I usually make the sizes of some panels (one with rebates on the edges) a little larger in the rebate width. Once the box is glued together, you can flush trim, and therefore ensure the joint is perfect.
