Brewster Buffalo,
RAM is a case of supply and demand.
Prior to the emergence of Windows Vista there wasn't much need to have more than 1GB of RAM in your machine unless you were doing something intensive, since a tuned copy of XP would run quite happily within 1GB.
Come Vista & co, and there's been a general trend towards 2GB becoming the new minimum. Hence the prices have dropped as volumes have increased.
Having said which, RAM hasn't been particularly expensive over the past decade, and I would suggest you were potentially being overcharged if someone asked you for £40 for 512MB of RAM for an average desktop/laptop machine. But then I don't know the specifics of your situation, so I may be doing the seller an injustice.