Dotcom-Monitor offers per-hour billing for using Load Injector Servers (LIs): 

Load Injector Hours  = LI1 Time + LI2 Time + … + LIn Time + Reference Server Time (in hours)

LI Time is a server operational time calculated from the start of the test on a server until the end of the test.

For on-demand tests, the actual Load Injector Time will be rounded up to the hour due to the mechanism of LIs operation. For example, a 20-minute test run on 10 Load Injector Servers (including Reference Server), the Load Injector Hours = 10 × 1 hour = 10 hours. So you will be charged for 10 Load Injector Hours.

In the case of SmallMedium, or Large plans, the actual Load Injector Time (test duration time) is used to calculate Load Injector Hours. For example, a 20-minute test running on 10 Load Injector Servers (including Reference Server), the Load Injector Hours = 10 × 20 minutes = 200 minutes or about 3.3 hours. In this case, 3.3 Load Injector Hours will be written off from your resource wallet.