As with any firearm value, it depends on rarity, current interest in the gun, and condition.
I have 'a few' Mannlicher-Schoenauer rifles. I have paid as little as $400 for a 1908 8x56 MS full stocked carbine with double set triggers, to as much as $1600 for a similar 1908 in 9x56 MS.
Luck plays a big part in finding deals, but your Cousin's rifle probably falls about in the $1000+ range if the condition is good. Did he say what he paid for his?