Not much. It was a common camera and many are still floating around in garage sales and the like. The zooming popularity of digital photography has driven down the prices for all used film cameras except perhaps for the really collectible stuff such as Leica, Contax and the German Kodak Retinas.

You'd be lucky to get $20 for the body. I don't know what lenses you have, but some of those might be worth a few dollars if they are top quality and a bit on the rare side, say a fast 300mm or a fast wide angle. If they are just routine items such as the 50mm standard lens, a 135mm f2.8 or a 28mm f2.8, they are worth little or nothing, even in good condition. If they are NOT in good condition, then they probably are worth nothing. The same goes for the body. If there is ANYTHING wrong with it, the value is zero. The fact that the lenses would be the old M42 screw mount hurts their value. Screw mount lenses always are worth less than the same lenses in Pentax K mount or other more modern bayonet mounts for the simple reason that almost no one uses screw mount bodies anymore, while the gear is a glut on the used market.

If the gear works and you have the interest, try using your granddad's gear. It will be fun.