The answer to this question is not singular. Because Roller Derby was the most prevalent roller skate maker, and then carried that success over into Skateboarding during the mid-to-late 1960s, there are a lot of Roller Derby boards out there.
The factors that will make a Roller Derby board worth something have to do with condition, and rarities in the item itself, such as black wheels vs "clay red" wheels, whether it is a longer board -- anything longer than 28" is more rare. Like most things that are collected, finding the right buyer for the right board might get you upwards of $500 for certain Roller Derby boards, most are going to go in the $10 to $50 range.
Hope this helps.
Mark
Maine Roller Derby was created in 2006.
Copenhagen Roller Derby was created in 2009.
Montreal Roller Derby was created in 2006.
Memphis Roller Derby was created in 2006.
Providence Roller Derby was created in 2004.
Greensboro Roller Derby was created in 2010.
Stockholm Roller Derby was created in 2007.
Roller Derby Brasil was created in 2011.
Roller Derby France was created in 2011.
Roller Derby Germany was created in 2011.
Helsinki Roller Derby was created in 2009.
Tucson Roller Derby was created in 2003.