As requested, hereâ€™s data from seven Explore matches played on either Warlord I (my gold bonus should be 250%), or Warlord III (300%).
Warlord I screenshots
Warlord III screenshots
From the screenshots posted so far, we see that Normal/Warlord I/Warlord III award 0gold values (with no 4+ matches taken) of 457, 508, and 609 gold. This is consistent with a base amount of 202 gold (yes, the 2 is there; no idea why), multiplied by 225%, 250%, and 300% â€“ exactly what I expect from the map screen.
The 0gold values do not work out as nicely if I use the multipliers from inbattle 4+ matches. (More on them later.) If I use those numbers, the 0gold formula works out to be 1.27*$(gold bonus)26. So you get a bonus based on your armor, but then 26 gold is subtracted at the end â€“ the only place Iâ€™m aware of in the game, besides select spells, where gold ever has a minus sign associated with it. Iâ€™m going to call this unlikely/wrong, especially because the rounding doesnâ€™t work out quite right.
Now, if I start earning gold during matches, what do I see at the victory screen?
Tables of multipliers
Normal (repeated from OP)

0gold 
6 
27 

457 
479 
560 
Mult. 

370% 
380% 
Warlord I

0gold 
26 
32 

508 
618 
643 
Mult. 

420% 
420% 
Warlord III*

0gold 
6 
26 
88 

609 
639 
741 
909 
Mult. 

500% 
510% 
340% 
The multiplier used for gold at the Normal difficulty is 380% (the 370% in that row may be roundoff error). The multiplier at Warlord I appears to be 420%. At Warlord III, I get 5 gold afterward for every 1 I earn in the battle â€“ a 500% multiplier! Sort of.
Check out that last column of the Warlord III table. I earned 88 gold just from 4 and 5matches. Iâ€™m actually pretty proud of that, despite it being the RNGâ€™s work and not my own. For my efforts, the game gave me a 340% multiplier â€“ less than what I was earning at Normal difficulty!
In conclusion, I have no idea whatâ€™s going on with the gold multipliers. The base (0gold) amounts are consistent with my armor/VIP/difficulty modifiers, but the gold earned inmatch is just crazy.