Hi Thomson,
It really depends if you test the SRAMs serially or all SRAMs in parallel. If you test them serially, the test time will multiply by the number of instances. If you test them simultaneously, you need to add some extra BIST logic, but the extra logic is not too large.
Most BIST insertion tools these days (LogicVision, Mentor Graphics, etc.) allow you to build a BIST controller that can test all instances simultaneously, so if you can afford the extra logic, it will save you in test time, and hence test cost.