通常租舖應該壓佢幾多價先啱?

It really depends on;

1)　How much is the rent? Is it in a good location?
2)　who is the landlord/ shop owner? (a private landlord or management office of a shopping mall?)

example: it is easier to bargin for a discount from a private landlord than a shopping mall. For a shopping mall, rent rate of each shop are almost fixed by the management. It's hard to bargin.

If the rent is 15,000, u might ask for 14000 from a shopping mall. and ask for 12000-13000 from a private shop owner.

this is my personal experience