Hi, in June I gonna visit Montana and South Dakota, staying in places like Billings and Rapid City. Since quite a while I'm looking for a Winchester 73 .44-40 in very good condition. How is the chance to find one there, how much does such a piece cost?

$1,000 - $25,000.

Probably find one for around 2 - 2,500.

It has been my experience that lever guns are cheaper and in better shape in the east than they are in the west. the guns in the east are used for two weeks in the fall for deer season then they spend the rest of the year hanging on a wall or in a closet. The guns of the west were used hard and quite often carried year round. Ten years ago I bought a bunch of 94s in 30-30 in the east for an average of $250.00 sent them west and tripled my money..
Good luck in your quest and enjoy your travels....