I had one HF crescent blade that still actually has some teeth on it but 90% of it is worn smooth. I have two Imperial and a handful of HF blades all the same usage that didn't last nearly as long.
Cutting nails generally is the 'metal' these are referring to, ductile rather than brittle iron like bolts. I have cut through several 1/4" and 5/16" bolts but anything bigger you got the wrong tool for the job.
As far as cost per cut, generally the comparisons out there are going to be wood with nails in it. Usually you find the very expensive and the least expensive are going to give you the best value in terms of cuts/$. Generally I only buy HF, Imperial, Bosch, or Fein. I have a few of other brands if they were on clearance, but otherwise that's all I've got. Dremel, Skil, and Bosch blades I have seen look very similar, and you may wind up with one or two of them being the same blade attached to a different mechanism but most of the Bosch blades are one piece of metal so don't count on too much overlap on those. haven't been impressed at all with the PC/Dewalt ones which are pretty much the same thing.