What you want to avoid with this sort of AB testing is the same person seeing multiple prices. The easiest, decently close approximation is just to split the page served by geocoded IP. The regions should be split randomly up-front.
Also don't underestimate the value of the more expensive plan to making your $4.95 plan look cheap. A simple answer to what could be premium is contained in your terms: Make the $99/month plan suitable for use in a production environment, as web middleware or a backend, etc by relaxing restrictions and having some sort of SLA.
