Bulk purchases of any iOS App are possible through Apple's Volume Purchase Program (VPP). This allows you to purchase any number of licences for an App without having to go through the main App Store.
1) Head to https://www.apple.com/business/vpp
2) Sign up to the VPP.
3) Login and search for "Site Audit Pro"
4) You can now purchase as many licences of Site Audit Pro as you need.
Note: This will only work for the iOS version of Site Audit Pro.