Since you only will accept "best" or "lowest cost" then this will be a moving target as one model is priced lower or another new model appears.

Have you looked at the HP K-series OfficeJets?

"if the color cartridge is depleted, the printer will still print in black and white."

That may be the deciding requirement. I don't know any printer that does that so when you find one that does that, it becomes the solution.
Bob