blackcat, I don't think any of us here can tell you how much "just a WISC" would cost unless we knew we lived in your area. To be honest, with a child with ADHD, I'd rather have a comprehensive neuropsych eval for two reasons: first reason is that in advocating for accommodations for our ds, having that neuropsych eval report gave us the backbone we needed when advocating in a situation where the school staff told us our ds didn't need "official" accommodations. Second reason is you have a child who did have a large discrepancy in verbal vs non-verbal on the CogAT - what if you have the school test on just the Weschler non-verbal and it comes in much lower than you expect is should? Or what if she was given the full CogAT or some other full ability test from the school and you saw a large split again? Your understanding of how her ADHD might have impacted her scores makes good sense, but what if there is any type of doubt at all or any possibility that something else may have caused the split in scores? For those reasons, I'd want private testing - where you know you are going to be able to ask questions of the person who tests and where you feel comfortable with the test setting so you feel like your dd has a chance to show her true abilities.
If your medical insurance covered your ds' neuropsych, you might be able to get a neuropsych eval covered for your dd too - I would ask for a referral based on the ADHD diagnosis + split in previous verbal vs non-verbal testing.
polarbear