Scrypt's maximum strength to increase entropy of lame passwords
You have to test it on the hardware you'll be using. I tried CryptSharp's implementation with a cost of 262144 and it took about 7 seconds.
The reason it costs more is cause of the memory it uses, and my process that was running the Scrypt was eating up 340ish MB.

How much of that was from the KDF? Don't know.

How would my box handle 100 people authenticating at the same time? Very good question.

This is something you need to performance tune to specific hardware.