Samuel, the scalability will depend on a few different variable. How many overall instances will you be monitoring? How active are your instances? Are you planning on hosting the application server and repo on the same host or separating them? How much resources will the host(s) have?
As a rule of thumb, you can monitor around 200 instances from one installation of DPA (results may vary depending on monitored instance activity). We have a feature called DPA central that will allow you to link multiple installations of DPA (a dashboard of dashboards). One thing to mention is it may not be scalability as to why you would want multiple installations of DPA. We recommend placing DPA as close to the monitored instances as possible (same datacenter is ideal). So, if you have a geographically diverse architecture, you may consider site installations. If you have specifics about your architecture and number of instances, we can certainly get into a more detailed discussion about implementation.
Regarding pricing, DPA is licensed by monitored instance. If you want specifics about pricing, I can certainly get you in touch with an account rep here at Solarwinds.
Thank you. I realize the answer is the typical "it depends", but I appreciate the information! This is exactly what I needed at the moment.