The most expensive Arena League promo I could find is the Island from 1999s Urzas Saga
P/T Number 431 Language English Legal Formats Alchemy, Brawl, Commander, Competitive Brawl, Historic, Legacy, Modern, Pioneer, Standard, Timeless, Vintage Rulings (2/1/2009) "Protection from everything" means the following: Progenitus can't be blocked, Progenitus can't be enchanted or equipped, Progenitus can't be the target of spells or abilities, and all damage that would be dealt to Progenitus is prevented
Dionus, Elvish Archdruid Elf decks go wide and small, requiring buffs to cross the finish line
Set: Invasion Increase: +$9.36 Current Market Price: $11.24 Vile Consumption has a similar story
This is done mainly to weaken them as removal spells or even sweepers
However, high quality doesnt always lead to high pricing