These things give the most money in the game when traded to the Maniac in Undella Town. The most expensive, THE (as in just one) Relic Crown, gives you $300,000!! The cheapest, Relic Copper, gives you $1,000 each.
Relics From Abyssal Ruins
P.S. Here is a Map of the Abyssal Ruins
Map of Abyssal Ruins
P.S.S. You also get 14 of Arceus' 16 plates here.
Arceus Plates 14 of 16