Hi, Ross!
Yeah, I agree. $239 for RCBS dies to reload 5-in-1 blanks is a bit rich for my blood too!
Those old 5-in-1 blanks were intended for .38-40, .44-40, .45 Colt, and a couple of other pistol/carbine calibers I can't remember.
If you can get hold of some 5-in-1 blank cases, reloading them w/o the RCBS dies should be no problem. Use a regular (.45 Colt) sizing die, and a modified bullet seating punch to press in the card wads. Charge with a full case of FFFg and compress moderately with a couple of thick card wads.
The problem with making up your own .45 Colt blanks is finding a rimmed case that is long enough and with a .45 Colt size rim and base diameter. The base diameter doesn't have to be exact, but the rim needs to be the right diameter for reliable feeding and extraction.
If I was to make up some economical blanks, I'd look at the .444 Remington Magnum case. It's certainly long enough, and the rim/base diameter is the same as .44 Remington Magnum. Now the question is, will it feed and extract reliably enough for you?
I dunno. It's worth a try!