It's practical value won't be much.  As a modern, useful bowling ball, it would be little more than a spare ball, and you can get a brand new one of those for as little as $50-$75.

 Now, as a collector piece, it is going to be worth more, but finding the right collector will be the problem there.


 In truth, it is "worth" whatever someone is willing to pay for it.  In a case like this, it is better to price it too high, because you can always come down.

 Don't know what you gave for it, but unless it is EXACTLY what someone is looking for, getting a big price for it might be hard.