Cost of Blowfish vs. Time

I use bcrypt / blowfish in php, and when I set the cost parameter to $ 10 (1024 rounds, I think), the encryption process takes 0.1 seconds. If I set it to $ 12, it will take 0.3 seconds. My question is: Is it taking 0.3 seconds of processor time, if I have 100 users performing this process, all of them will have to wait 30 seconds (0.3 x 100)? (editing: may be shorter due to dual0core / multi-thread processing, but even 10 seconds is unacceptable).

Also: what is a good value to leave this cost parameter? some people recommend $ 16, but it takes more than 5 seconds on my site (hosted by a large web host).

by the way, I use the following code to check the time it takes:

<?php
// set a password to work with
$var1 = "doodoo1234";

//echo that password onto the screen 
echo $var1 . "<br /><br />";

//Start the clock
$time_start = microtime(true);

//Run blowfish function to encrypt the password
$hashedpass = PassHash::blowfishhash($var1);

//stop the clock  
$time_end = microtime(true);

//echo the password to the screen
echo $echohashedpass . "<br /><br />";

//Echo the length of the encrypted password to the screen
//(this taught me that blowfish always returns a 60 varchar string)
echo strlen($sajpass). "<br /><br />";

$time = $time_end - $time_start;
echo "that took $time seconds\n";
?>