B&B is family operated. In most cases, you will be sharing a house with a local family. Rooms are cleaned darily by maid.
Montreal has a big Chinese community. Try http://www.sinoquebec.com/bbs/ and you'll find B&B or room for short-term rental at really good price.
Airfare is a killer. Remember August is peak season. My best guess is 1500 -2000 CAD, around 10000-12000 RMB
Let's say you come for a week: airfare(1700) + stay ( 100 dollar/day)+food( 35 dollar/day)+transpotation ( 10/day)+ Visa Application ( 150 dollar)+etc.., you can easily blow away 3000 dollars ( 20K RMB).
If you need visa to come, that will be another story-- not that easy to get short-term visitor visa.
Your guys don't have to rent a car in Montreal. Stay in downtown area and use subway (metro). That's the best way to get around the city. Both Mcgill and HEC Montreal have metro stations nearby.