<p>I wonder if there is any good guideline for system administrators to calculate the resources needed to host an ASP.NET website? I want to do some estimates on the processing power needed so I know what kind of VPS/Cloud I need to get.</p>

Load testing will be your best solution.
Applications differ in how they scale depending on a large number of variables including types of problem solving, approach to solution and other factors.
100 concurrent users consuming 10% CPU does not mean 500 concurrent at 50%, it could be 20% or it could be 100%.
Your bottleneck could be processor or memory or network bandwidth.
Without actually placing the load on your application, any estimation will be merely a guess and will quite likely be proven incorrect.