I run a small lab, and much of my work is scanning and processing. If you are looking for rigorously clean negs, you want to do it yourself or use someone like me...i.e. a very small scale lab which processes by hand and one-shot. Also someone who is scrupulous about cleanliness in the lab. OR, you find a very high end professional lab that runs lots and lots of film and still maintains a lot of cleanliness. There are a couple of reasons for this -- in small labs and minilabs, the film processing machines tend to use roller-transports. The film is dragged across rollers that can pick up dust and grime over time. This is particularly common when you bring 120 film to a lab that mostly processes 35mm -- the 35mm track will be mostly clean, with gunk on the edges...when you pull the 120 across it, will pick up all the gunk that built up around the edges of the 35mm film path. This is solved by lots of cleaning, which most labs no longer have the volume, manpower or inclination to do. Also, most larger labs use replenished solutions -- they are using the same chemicals over and over again. Over time these get dirty. There are ways around this, but it is still an issue even at good labs. Then there is of course the matter of handling. The less the film is handled and the cleaner the environment the better. Ideally you want filtered forced air drying with a clear exhaust path.
The best way to avoid dust is to do it yourself -- use distilled water when you mix your chemicals, and at least filtered water when you process, and especially use distilled water with photo flo/ilfotol at the end before drying. Try to avoid rugs, carpets, towels, fuzzy sweaters etc when in the lab. Vacuum and clean regularly. Use a hepa filter if you have one. Dry it in a drying cabinet...if not, find a place that has very little traffic, such as in a shower in a bathroom that you do not go into until the film is dry.
For black and white, I charge around about 10 dollars a roll for hand processed black and white (usually in a Jobo), but that includes a 25.5% VAT. I can't charge too much because so few people even do it here anymore. I have given up processing E-6 and C-41 because doing it in a Jobo requires lots of work (about an hour per process, which can only have up to four or five rolls or 10 sheets of 4x5) and 5L of the chemicals costs over 125 dollars here. I simply could not charge people enough to make it worth my time, so I gave up doing it.
On the matter of scanning, I also do that with a Hasselblad X5. Dominique is correct in that the X5 is quite easy to use, but the majority of your time is spent loading and cleaning the film, setting up the scanner and then doing the subsequent editing. Clients very rarely want FFF files because they can't really do anything with them unless they are very computer literate or they have an Imacon/Hasselblad scanner already. Everyone just wants TIFFs. If they say "match the slide" or hand you a print, that requires a lot of work in both FlexColor and Photoshop. Other times they just want a rough scan that is easy for them to work on later. That requires a lot less. As such, I do all my scanning on a quote by quote basis. If you just want a flat, basic scan with no work, the charge could be as low as 8 or 9 dollars for a full resolution scan. I do not charge differently for different resolutions because they really do not make much of a difference in the workflow...the X5 is so fast that if you scan at 1000 dpi or 8000 dpi it takes only a minute or so difference. Most of the time is in the scanner's loading, and pre-scan gymnastics. A 500mb scan takes about 2 minutes. If you want to match a slide or have a file that has been cleaned up and worked on intensively, then it is going to cost much much more. I would love to be able to charge 50 dollars, but the market here simply does not exist for that, so the best I can really charge is around 20, and that is for a scan that takes 30 minutes to an hour of editing. But since it is my own business and I am the only employee, I am basically slave labor!
Scanning is much better for the bottom line than the film processing though...as much as I enjoy it, I barely break even on each roll I process, and I certainly wouldn't if I had to have any employees. For example, mixing a 1L working solution from the 7 step E6 process takes about an hour, hour and a half if you do it as carefully as it needs to be done. Then you have to do the entire process at +/- 0.1 degree C at 38C. The process itself takes about 45 minutes to an hour. The chemicals in a 1L working solution cost 25 dollars and expire in one week, and can process about 15 rolls. Then clean-up takes about an hour. So if someone hands you a single roll of film and wants you to process it, you start out in the hole about 3 hours of attention-intensive labor and 25 dollars. It is not a good proposition most of the time! I only really process color for myself anymore...