Unless the situation were unusual, I wouldn't pay for an audit for a PTO/PTA. The groups I've been involved with either get a CPA to donate the time or (and this is very common) have an audit committee. The committee is usually 3 members and includes no one responsible for financial transactions (i.e., not treasurer or president).
The audit committee on which I served was for a PTA chapter and they had a very specific checklist of items and good template. I also think there have been some documents posted in the file section of PTO Today. The PTA is a high school chapter so probably only 10-20% of the funds/transactions of our elementary PTO. Very easy to audit. The three of us met at Starbucks and completed the audit in 1-2 hours.
As I recall, it was a combination of financial and non-financial activities:
- verified the math, numbers
- reports/checkbook match bank statements
- verified all checks had two signatures
- all expenditures properly documented
- all expenses documented in meeting minutes as approved
- all revenue properly documented
- dates of deposits appropriate for when money received
- sales taxes paid
- IRS filings completed on schedule
It really wasn't hard. Back when we were in elementary, our school district accountant would do the audit in June/July. We had to leave everything with him for about 4-6 weeks so he could fit it in. With 20+ campus locations, our district is pretty serious about parent groups being separate/independent, so I'm not sure exactly WHY he did this for us. We had been through an embezzlement and he helped with the investigation. For several years after that he just did the audit for us. (Very helpful, it wasn't a suspicious/negative thing.) He couldn't possibly had been doing it for all the campus locations, but it was nice for us!
Barring a problem, the audit report isn't intended to be punitive, even though it's unlikely you'll get everything perfect. There's usually a small list of infractions - timing, lack of signatures that may be mentioned. But that's just areas to improve - PLUS it provides good ammunition when members want to cut corners. "Oh no, we can't do that. The auditor will ding us when he reviews the books."