I cannot write in Chinese as my pen power is out of order.
I do not consider the accomodation in Marais expensive. I had stayed in apartment and small hotels which cost several hundred dollars a night.
For a group of five, I recommend Le Fourcy, which is 3 minutes' walk from St. Paul's station (metro). It is converted from a nice old building. It operates like a hostel. But it has clean and decent double, triple and quad, with attached bathroom. If you get a quad, you will be paying the cost of a dormitory bed per head. Breakfast is provided.
Most of the people who stay there are family, boy scouts and young students. The only limitation is age.
I have just found their website
http://www.hostelz.com/display.php/67+Fourcy
If you arrive in Paris at the beginning of the week, carte orange (a 7 days' pass) is a good choice. It is less than HK$200 and you can use it from Monday to Sunday on metro and bus. You need a photo for it.
There are also other types of day pass but they are relatively not good value for money. I would rather buy a single journey ticket - remember buy to in 10 each time, you get a 50% discount!!