Godfrey... You didn't answer the question. The point is the relative bang for the buck of the K5 and Ltd primes vs An M9.
The question is the value of that relative bang for the buck. And it is unanswerable in a general case.
There is a significant difference in the output of these two cameras. How to value that difference is what's at issue. Whether it influences the salability of photos, whether it improves the aesthetic experience ... etc ... all these things have to weigh into the context of that evaluation.
The ultimate question is "how good is good enough?" and every one of us, from the entire context of our use of this equipment to make the products of our labors, has to determine that for ourselves.
There's a photographers I know who owns $100,000 worth of photo gear. He makes pretty pictures. Arguably, you, using a $1000 camera, make better pictures. He's happy with the equipment, it makes him feel good, and he likes the photographs it produces. You're happy with your equipment, it makes you feel good, and you are happy with the photographs it produces. Is his $100,000 worth of equipment better or worse than your $1000 worth of equipment? Likely, technically, maybe.
But in the end, the question is irrelevant. This isn't a class in economics, of the Millsian utilitarian theory ... the greatest good for the least expenditure.
To an artist, no tool is ever good enough, no work ever really right or finished.
Artists are perfectionists, and perfectionism doesn't rest on "good enough" or "relative bang for the buck".
One of the magic tricks learned by the great artists is when to say "I'm done, it's good enough", in every context. So that they can go on to the next work...