both my 890 and baby R have machined primary casings,not sure of total cost because both were done during full rebuilds,i think the machining costs were about £80 ish Butch
JHP used to do an exchange. I remember paying £60 some two years ago.
As to crud... well I guess you let crud out (have you seen the inside of closed cover?) as well as risk crud in. However, I ran a 996 for over 20K miles with a machined casing and had no problems. It is arguable whether it is worse than a vented clutch cover. I am not sure it matters. It looks good and will ventilate the clutch better.
there are lots of what if's on track. like what if some plonk takes you out your bike flips 10 times and ends up in a pile of bits the other side of the track then catches fire. I don't tend to think about these possibilities that much. maybe I should maybe I shouldn't :P