Actually CelGen, you did well, even if you can't use this one for your 10MB ProFile.  It is a collectible chip at least.   The two I bought I got in the $20 range also.  You just have to be patient.  I believe I found mine by looking at the photos posted for Z8's that didn't list the part numbers properly.  If the ad has the part number, they will not go cheap.

To provide further details, these are known as "Z8 protopacks".  From http://verhoeven272.nl/fruttenboel/softy//z8techman2.html

The Z8 protopack allows the user to prototype systems hardware and develop software that is eventually to be mask-programmed into the on-chip ROM of the Z8601 or Z8611.

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Very early 5MB ProFiles had the Z8 protopacks in place with running ROM on EPROMs.  When the Z8601's were finalized, they came with those and the "piggy back" chips were no longer used because they were more expensive.  Low Level Format versions were never produced in final versions because the software on ROM should have never left the factory.  Thanks to the code being released and software called BLU, we can re-low level format our ProFiles.

I'm not sure if I've ever heard of early Z8613's in 10 MB ProFiles.  As I understand it, 10MB ProFiles, by themselves, are hard to find.

The obvious difference between the two piggy back chips is that one utilizes 2k and the other utilizes 4k in "on chip" (literally) memory.   My "2K" image is a 4k chip with 2k copied twice and burned as a 4k image.  I don't have something that will burn 2716s yet.