Agreed that there are lots of variables in play, and that the start-up costs for a typical neuropsych practice are going to be higher than for a therapy-only practice. As was said, you'll needing testing materials, a physical office space and everything that comes with it, and then all the other practice stuff (e.g., scheduling software, EMR, phones). If you need to take out loans for that, you'll need to factor the interest and repayment into your operating costs.
All that aside, you can probably make a 100% clinical practice viable, but it becomes easier with testers. You can pull Medicare numbers for your area online and use that to give a rough estimate of what you'll earn for a typical outpatient eval based on CPT codes used.
I pulled data on this in my area maybe 5 years ago now. Based on that, assuming I know how to use a spreadsheet (debatable), it worked out to about $40-45k/year per weekly eval that you do, assuming you work 48 weeks/year (and based on doing all your own testing). So if you see 4 evals/week, that's around $160-170k/year gross, and for 5 evals/week, it's a little over $210k/year; probably want to subtract like 10% of that for late cancellations, no-shows, and other random stuff. A tester adds almost that same amount, assuming you can actually bill the insurance provider(s) for their time (i.e., 96138/96139) and don't just have all those charges auto-rejected.
This is assuming you can quickly fill a practice. Which in most areas, probably isn't an issue if you keep your wait list at or under 2-3 months.
