from what I've seen . . . laser jammers can be pretty effective. but $250 for a used unit? you can get a new one that blocks radar and laser for that price
http://www.radarjammers.com
truthfully don't waste your money though. most of the time cops use the "instant" on for radar and laser and you have no chance against it.
and goose will tell you how unreliable these devices are
the only ones that actual work are so freakin illegal
$250 can go better toward something else . . . just don't speed like a crazed moron