Speaking to the Polish 'Krakow' overprints, I think it would depend on the source of the forgery. Many of them are printed offset, of unknown provenance, so I'd guess not much more than the market value of the underlying stamp, you can produce something similar at home on your computor.
If, however, you've got something printed typo (as the originals), 8.75-9.0mm high (as opposed to 9.0-10.65mm high), from the same font as the originals, then you've possibly got a Muller forgery, maybe a bit of a premium there but nowhere near the price of originals.
Very soon after the Krakow overprints were sold out (beginning of 1919), Muller got hold of the same linotype machine used for creating the formes for the overprinting. Each of the positions on the sheet is plateable by measuring the relative horizontal distance between the "Poczta", the "Polska" & the rhomboid in between them, so that was Muller's first problem & he didn't get it right. His biggest mistake was in getting the height of the whole overprint too short.
Lots more info on these isues & forgeries here-
http://polonus.org/Krakow/index.html