How much is a real double sided quarter with 1985 on one side and 1995 on the other worth?

Why and how do you know it's real?

1. The mint uses dies that are keyed so that two fronts or backs can't be put into a press at the same time 2. What are the chances that a coin would leave the mint in 1985, find its way back 10 years later, and get struck with a new date on one side only? 3. Each year's dies are destroyed at the end of the run, so a 2-date combination can't be made by a rogue employee. Bottom line - you have a privately-manufactured magician's coin worth nothing to a coin collector.