Artisans work for the wealthy but cannot afford their own produce.

There are many variants of this expression. Those that refer to a cobbler are usually British in origin whereas the USA versions use shoemaker. These may be constructed as any combination of:

The (cobbler/shoemaker) always (wears the worst shoes/goes barefoot/has no shoes).

The poorly shod person may also be the cobbler/shoemaker’s wife or children or family. More widely, many expressions have been cobbled together in the form of “The ‘X’ always has the worst ‘product of X'”.

For the definitive version we need to find the earliest example in print, which is in John Heywood’s 1546 glossary A dialogue conteinyng the nomber in effect of all the prouerbes in the englishe tongue:

But who is wurs shod, than the shoemakers wyfe,
With shops full of newe shapen shoes all hir lyfe?