Which would be cheaper??

To install a series 3 carrier with the 3.42 gears...

or

To install the 3.42's for the series 3 with the spacers for the series 2 carrier?

Basically, is it cheaper to get gears or a carrier installed?

I'm sure you can easily come across a used 3 series carrier with 3:42's. Since the majority of T/A's came with them. Check out ebay.