Hi,
1.The 50w Raycus fiber laser is powerful enough to cut 0.8mm Brass/Silver
2.It will be depend on the size of your file.If the file is complicated or large,it will need more time.I used to cut brass 1mm with a 50w and the size is around 30x30mm.It takes more than 3 hours.0.8mm will be faster.A smaller lens would be help
3.The price range for a 50w is around $7k for a Chinese machine and it's just a reference price.I also heard some supplier offer $6000 include shipping but I doubt about the quality.
For the jewelry industry,Raycus fiber laser is the good optional and I don't know some good alternative because the work need accurate cutting.
You can get a 30w fiber laser is your budget is $5k and it can cut through 0.8mm brass/silver too but it will need more time.
One of my friend have a 30w fiber laser and cut 0.55mm gold at speed of 7-8 min
Tim