TNeuschwanger,
I have not found the 500 VU limits in the documentation, only very vague wording "This depends on the license". Multiplying every 500 VUs by 6500 Eur seems unbelieavable. I am long-term fan of ReadyAPI and SoapUI, but such prices are not aligned with the benefits. Even 6500 Eur for unlimited VUs would be way too much.
JMeter is free, and offers unlimited number of VUs with the option to run tests on multiple machines. JMeter is much stronger in performance testing in terms of functions and simulating real load. JMeter also supports Groovy scripting.
The strength of ReadyAPI load testing is in re-using the functional tests. And I appreciate launching a load test with two clicks. But, it works for very basic tests only.
When I design a real performance test, i.e. simulating real load I need to create a dedicated test anyway. In summary, JMeter is far cheaper and more efficient for such purpose and you don't reach almost any limits in your scenarios.