Given that Ragnar's raid would have happened like this in real life ... is there any reason why King Aella would not have given Ragnar only silver / any reason why Ragnar would not ask for "2000 pounds in gold"?
Gold is usually more valuable than silver, but would there have been an advantage to having both gold and silver in this time period (~9th century)?
Put it simply, silver had became a kind of standard currency in north-western Eurasia both in Europe and in Scandinavia since the end of the 8th century partly thank for Charlemagne's monetary reform in the kingdom of the Franks. Thus, silver would probably be a most commonplace symbol of wealth for the Vikings in raiding or in their homeland, as testified by the contents of various hoards in Viking Ages, such as Spillings Hoard in Gotland (linked to the news article in Swedish with pictures of silver coins and armbands).
As for the political as well as economic significance of silver in Viking Age Scandinavia, please also refer to my previous post in: After a successful viking raid, how did all the riches change their life and what did they do with their new found wealth?