I would guess that the cost of owning a 4.3 vs a 6.2 "could" be about even.
I get very similar mileage to my Mom's '93 S-Blazer (with my '84 6.2L Burb). The 6.2L is cheaper to maintain than that finicky CPI engine though. I have much more cargo space too.
The 4.3L S-Blazer didn't come with 3.73s until '95+. 3.42, 3.23 or 3.08 on 94-older.