I love working in colour but occasionally get asked to produce finished drawings for framing or reproduction. Some collectors, particularly the military enthusiasts, have a nostalgic hankering for the kind of drawings that were common in books and magazines and comics before reproducing colour artwork became more affordable.
My favoured approach is to treat them as seriously as a painting, often with research, live models and real equipment then Photoshopping the raw elements into a coherent whole for client approval first. This takes a lot of time and, although the finished item may be smaller than a painting, they can't be created particularly cheaply this way. Nevertheless, often a monochrome is easier to reproduce in bulk, especially in the kind of hobby magazines, niche market wargaming books, or advertising that some of my clients seek to achieve.
As you can see from these images the photo mockup is scaled and then rendered in pencil before being finalised in fine liner pens followed by brushes with Indian ink washes. The black and white image can be used in many different ways for reproduction - maybe a sepia version produced in Photoshop, or vignettes from it cut out with feathered edges to produce start or end illustrations for chapters. In this way the total price can become more cost effective.
Another "saving" can be made as some of the drawings turn out so successfully that the client decides to have an acrylic or oil painting based on it. As much of the ground work has already been done it makes the painting a more economical purchase. The drawing for "British Light Infantry at Winnsboro 1780" became an oil painting "A sympathetic hand.