Anything less than $500 for a reproduction 50 round drum would be a good deal. Generally they are about $500-$650, but for a New York 50 round drum, these are running between $650-$1100.
Of course, it all depends on condition.
The 100 round drums are a different story.
The original Colt 100 round drums run between $3000-$5000. If you find one in good condition less than $2500, buy it.
The reproduction 100 round drums have been running between $800-$1200. This may sound like a good deal, but you would consider yourself extremely lucky if you found a reproduction 100 round drum that actually worked.