I have always been curious: is there an actual market for original viols?
I've recently seen a few of them in a museum, and there seemed to be no standard.
Hmmm... very complex question and you can earn a Ph.D studying it....
There are two groups of viols: renaissance and barqoue. For the purposes of brevity, I'll ignore renaissance viols as that is a minefield. Baroque viols were an evolutionary improvement on the renaissance viol - they are built slightly differently, they have sound posts (like a violin does) etc. The shape is more-or-less standardised. The standard sizes are treble, tenor and bass. There are other sizes: the smaller pardessus, and the violone or great bass (in fact the ancestor of the doublebass).
Unlike the members of the violin family (which were developed later and which are not related to the viol) there isn't really an agreed standard for size/pitch. Modern convention is bass in D, tenor in G, treble in D and later the 7 string bass in A (mostly French).
The big challenge is very few original viols have survived. A lot of bass viols were butchered and turned into weird cello shaped objects. English viol makers were regarded as the best with makers such as Jaye and Rose being amongst the best regarded.
The treble and tenor viol fell out of general use in the early 1700s. The bass viol continued (as the bass continuo in baroque music) but finally peters out around late 1700s (Abel is usually regarded as the last composer to write music for viol - sonatas for bass viol in 1770). The violone loses 2 strings and becomes the doublebass.
So, unlike violins/violas/cellos which have been in continuous production, new viols were not being made after sometime in the 1700s. So the only old viols are generally in museums (there are a very few in actual use). So, unlike violins etc just about all viols are new modern instruments. Modern instruments are usually based on a model such as Jaye, Rose, Norman etc (and to be fair violins and cellos are usually modelled on a classic instrument). There are no 'mass produced' viols. The cheapest generally available are semi hand produced ones from Czech republic and Chinese (which are usually fully hand-made). A Czech or Chinese tenor/bass will be around £2,000, treble rather less. The cheapest hand-made commissioned instrument in the UK would be about £3.5k from a young maker starting out and probably £4.5k - £6.5k from an established maker with a 'name'. Lead time usually 6 - 12 months, more for a 'name'. You can get slightly cheaper 'student' instruments which are less fancy (simpler purfling, no carving, no rose etc).
My tenor as seen in my picture here is a baroque tenor viol. It is a Czech one and is based on a John Rose instrument from 1604 which is in the Paris Conservatoire. Chinese instruments have only been available in the UK for less than 10 years and weren't around when I bought this instrument. The base standard of the Chinese instruments (these are badged "Lu-Mi" which is actually made up from the name of a well-known Finnish viol player who has set this company up) is very good but they are somewhat variable. Good instruments are really very fine and compete with luthier commissinoed instruments.
Sorry for the long (and off topic) post!