Typically high hundreds / low thousands for a used GPSDO with a CSAC.

And Americium is not as useful for a timing reference, as it's not as stable as Rubidium and a lot less safe to handle. Otherwise time nuts would hoard cheap smoke detectors :)

I have 2 Rb GPSDOs and a few more OCXO ones, but had never heard of CSAC modules and thought that the inevitable next step would be an HP 5061A Cs clock or later model.

So now you have me going to eBay in search one but all it turns up are BM25CSAC carburetors! What are the magic keywords to use in my search?

Americium is also chemically different and may not be as practical as cesium and rubidium to ionize and trap.

Words of wisdom (from a time nut).

Spinthariscope https://xkcd.com/2568/