I can't speak for the auditors, since I am a business attorney, but I do know what I have seen in helping to find this type of service for clients. For example, just within the past year, I was asked to help find a good auditing firm by one closely held Silicon Valley startup with fewer than 50 employees and a few million in annual revenues - the cheapest estimate came in at the $60K-$80K range (and this was from regional firms, not Big Four).

Don't know exactly why this is, except that the auditors themselves kept referring to Sarbox-type requirements that had become standard practice for all audits, public and private. Also, I think many of the smaller players have been chased out of the auditing field by the new complexities injected by Sarbox and relating recent developments in the field. Whatever the cause, at least in and around Silicon Valley, it is no longer easy to find a small firm that will do a quickie audit for a modest sum.

[Edit: just saw reply by tptacek - he says it much better than I do.]