The 1889 model of schmidt rubin uses cases that are 53.5 millimeters long and the GP90 is the correct ammunition to use in it.
Do not shoot the more modern GP11 which is intended for the newer models such as the 1911 and K-31. These rifles have the locking lugs in the front of the bolt (1911 and K-31). The 1889 has the locking lugs set further back and is intended for the lower pressures of the GP90 ammunition. The GP11 ammunition is loaded to higher pressures and if fired in a 1889 model damage to the rifle and possibly the shooter can result. You can use the 7.5x55 cases trimmed back to 53.5mm and lower pressured handloads.
I don't have the time or tools to load my own ammo. If anyone does, this would make a nice add to your collection.
In 2005 they were going anywhere from $395-$600. I'm going to ask $500 obo.
Great. Now it's not an auction. If you want to see what a current price for your rifle is, take a look on Gunbroker. Don't just look at what they are advertised for. Go to the advanced search and sign in. Then look in the "completed auctions" tab and look at the ones that closed and "sold". Takes a little time but you get an idea what they sell for. Good luck.