In the fic (One Piece crossover) the Starks are all forced out of the country and end up joining different factions (Sansa joins Whitebeard and becomes very powerful, Robb joins the Navy, Bran and Rickon are taken to grow up safely in the East Blue) Arya joins the Straw Hats. But to give the details for this exact question.

Arya is captured by Ramsay Bolton. Taken in Winterfell to marry him. The Straw Hats go to save her. Get Separated. Ally with Cersei and The Frays. Luffy punches Cersei for suggesting they get Arya back without fighting since he wants to punch Ramsay. Straw Hats arrive at Winterfell day of Wedding. Crash it. Beat Ramsay. Arya leaves Winterfell in Ramsay's hands for plot reasons and rejoins her crew. Her wedding becomes this AU's version of the Red Wedding.

Standard rescue arc. For plot reasons the Straw Hats take the blame for the Red Wedding in order to protect everyone who helped them from being seen as allying with pirates.

Arya explains it here.

“Arya Stark and the Straw Hat Pirates organized the Red Wedding so we could steal all my House's gold. They laid waste to Winterfell and tricked the highborns of Westeros into aiding them with a false sob story about Ramsay being a Pedophile. Then they betrayed you”

Being pirates the cover story uses gold as the motivation. But what I'm trying to figure out is what would be a reasonable amount of raw gold/treasure for them to take. Not like Golden Dragons since I just am not good enough at math to figure out how much those are worth compared to One Piece money with is = to Yen IRL.

So I figured it be easiest to just have them steal raw gold they could sell for money. But what I'm trying to figure out is how much is reasonable?

Not sure if this makes any sense. Money and math are confusing.