What is the CI chondrite concentration of REEs/rare metals, in ppm/grams per ton?
You should start with the GERM Reservoir Database. Not the easiest to use website, but the most comprehensive. You can look up elemental concentrations in all kinds of "reservoirs", including meteorites. For example, look up CI Chondrites to see the elemental values that are commonly used for REE-diagram normalisation. You can look up by specific meteorites, or averages of groups. You can see values by paper, and in most cases they also include a scan of the original data from the paper itself. Note that it's also usually good practice to actually read the paper to understand how the data were collected, analysed and what do they mean.

That said, as for your general idea of using asteroids as ore deposits for rare metals:


  ...because I want to investigate the notion that asteroids represent abundant stocks of rare resources.


As a rule of thumb, this works well for the PGE (platinum group elements) and related metals (Te, Au, Re). The reason is that those metals are chalcophile or siderophile, and in the Earth they are concentrated in the core and depleted in the mantle and crust. In (some) asteroids, this differentiation did not occur and the concentration of these elements is still (relatively) high in the bulk material.

However, the REE and other HFSE metals (i.e. Zr, Hf, Nb, Ta), are not concentrated in the core. In fact, being lithophile elements, they are particularly enriched in the Earth's crust. Going to mine them from asteroids will be pointless.

Here are some more questions and answers that will clarify some concepts:

Why is uranium only in the crust, really?

What is the difference between chalcophile and siderophile elements?

What are the high field strength and large ion lithophile (HFS or HFSE &amp; LIL or LILE) elements?