Priest does not sell a fragment of Maiden's tickets.
Here are your US arenas of the Firepower world tour :
Michigan Lottery Amphitheatre at Freedom Hill
Soaring Eagle Casino & Resort
Northwell Health at Jones Beach Theater
Daily's Place
Treasure Island Hotel and Casino
Harrah's Council Bluffs
and so on...
The biggest venue they've played in SA was a Sao Paulo stadium which was one of the average venues of Book of Souls SA leg, where t Maiden played 60k+ venues, and sold out the said stadium of 43k capacity. If Priest done that there would be huge media coverage and not just some shy photos of stage and center field + some crappy phone pit bootlegs.
Book of Souls grossed over $100M, if you want to back up your claim find me a >$100M album tour from 'legacy' act that is not Metallica or any other newer act. (ACDC is a r'n'r band, they're as comparable to Stones as to metal bands)
That Metallica sells better than Maiden per album per tour that was always a known fact. They are popular and a sort of a 'national' band from USA although they don't have anything that USA specific to them which means they sell like hotcakes both home and abroad.
Motley Crue stepped in Europe a couple of times in late 80s early 90s and that was it.
Them being more successful live band is laughable. They are a stereotypical American hair creeps band, that shit does not sell well abroad. Slipknot although sucessful are not as Maiden successful and nor is Nightwish.
FWIW nobody in 'pure' metal world bar Metallica and Maiden can make a stadium gig in entire south Europe. Do you see Motley Crue, Slipknot, Nightwish, Priest playing sold out stadium shows here, in Portugal, Spain, France, Italy, and the entire Balkans + Turkey and so on. Nope. Metallica, Maiden. From affiliated worlds, AC/DC and Rammstein. That's it.