Building a propane forced air forge.  Would it be possible to use a pressure regulator ($25) vs. a squirrel cage blower ($65) to control air flor to the burner?

I have a large 2 stage compressor so air is no issue.

Thanks for any thoughts

Possible?  Sure.  Great idea?  Maybe, maybe not.  You're after low pressure high volume air for a burner, which is exactly the opposite of what an air compressor provides.   Now, if you use significantly larger diameter pipe to deliver the air and there's some distance between the air inlet and the burner, that combined with your regulator will produce the effect of lowering the pressure significantly.   However, in the long run it will possibly put more wear and tear on your compressor than the $40 difference you are talking about.

You can frequently find suitable blower motors for cheap or free.   I used the blower from a power vent water heater that had to be replaced when I started.  Others have used bathroom vent fans, bouncy house blowers, reverse flow shop vacs, hair dryers, etc.  Personally I prefer some of the quieter options since things like hair dryers or the bouncy house blowers are noisy and annoying fairly quickly.

Compressors cost more than a fan and have a limited use life before they need repairs or replacement.  Did you visit your local HVAC installation/repair place and see if they have any exhaust assist blowers for super efficient furnaces that were tossed on the bone pile when the furnace died for other reasons?  Make them a bottle opener and you could probably get a half dozen free.

I believe Brent Bailey uses or used shop air for his forge. A fancy regulator isn't needed, a regular gate valve as used for water piping would work fine.