B&W film is definitely the low cost option, unless you are going to shoot thousands of frames. Leica makes a version of their M, in monochrome, it sells for around $8,000.00. At $0.50 per frame, that’s a lot of film!
Maxmax.com does conversions, but they only do the hardware, not the software, so the camera still thinks it is taking a colour photo. This means it will not be any sharper than the camera before conversion, and you will lose the ability to have your favorite editor apply filters since the colour information will not be available. The whole point of a digital camera that does only monochrome is that photosites only record levels, not colours, so a camera that knows it is only doing monochrome can convert one photosite to one pixel, without fanfare. This gives the enhanced resolution. Most digital cameras have a Bayer filter over the sensor, and software figures out the colour for each pixel based on looking at several photosites. The exception is the Foveon sensor in a Sigma, which is a 3 level colour sensor, but that doesn’t help our search for monochrome. Anyway, a hardware conversion that does not replace the software still does the interpolation, so values for any given pixel are not the values for the same given photosite.
I thought I had seen a camera in the $1000 to $2000 range that did monochrome, from the factory, but I can’t find it now. If I come across it, I’ll let you know.
Almost all the dSLR’s have a B&W option which takes the raw sensor data, then makes a desaturated JPEG. The cool thing about that is by shooting raw, you can set the camera to B&W mode, and see the B&W version on the camera’s back when you review your shot. Since you have the raw file, your favorite raw converter can give you either a B&W or colour image. And, since the colour data is there, you can apply digital filters to make the sky lighter or darker, for instance. If you want to do that with the Leica monochrome or B&W film, you have to use a physical filter when taking the shot.
A couple of years ago, I shot a roll of colour print film and a set of digital images through the same Canon 24-70 and 100 mm L lenses. The digital images were much sharper than the film. I don’t like B&W enough to do the same test with B&W film, and I gave back the film body, but it would be an interesting experiment.