Going sailing worried about the keel falling off is no way to start. I'm assuming you did not get a proper survey or you would have put your mind at rest by now, either that or already have dropped the keel for repairs
The boat would have to be in pretty deplorable condition to be about to drop the keel. You can check for stainless steel bolts with a magnet (it shouldn't stick - although some low grade SSs can be slightly magnetic)
Catalina user group websites should be able to provide more information for your specific boat.
As for how much a new keel runs, I don't think West Marine stocks them .
Anyway get some good professional advice and a proper inspection.