You need to define a benchmark (mean, median, or any other value) for each metrics: number of annual visits, and money spent, to determine what is low and high.
I hope this helps you to reach your requirement.
The above work for Dual axis Color Scatter Plot is very helpful. Do you know of there is a version of that solution for Tableau 8? I was unable to open the link attached above.
Please be aware that you can install 8.1 concurrently with version 8.0 on your machine.