Is there a formula or generally agreed-upon rule for this... say the module is designed for a tenth level character, but doesn't supply the character with starting gold or items, just leaving that up to the player to "console in", import, or whatever?
Nope! Think of the difference between a level 10 character in Aielund vs Swordflight alone, for example.
In general, people who prefer somewhat lower magic are thinking "You have a +1 weapon most likely, maybe a +2 if you're lucky" while people on somewhat higher magic are thinking "You have a +3 weapon most likely, maybe even a +4 weapon." If you know more about the general magic level and resting intentions of a module you can hazard a better guess, but otherwise? This isn't even getting into the actual balance issue and cost of items -- something with 5 or 10 resist to all physical damage isn't very expensive price wise (9.4k for 5 to all, 33k for 10 to all) but incredibly powerful power wise. The price doesn't reflect the power.
Then you bring up other potential issues like Haste or immunities...Permahaste is less than 14k and Mind Spell immunity is only 10.5k.