You can contact a Telkom Business sales consultant, there is a specific product tailored for your kind of use (not advertised at present AFAIK, and requires signing a specific contract), but significantly cheaper than taking individual accounts. The product name is 'UBI' or 'UBR' (Usage Based Realm/Internet). It is not limited to Local-only, but AFAIK the administrator account can view the usage and administer the blended/local cap for each/all accounts from the web interface, and you are post-billed for the total local+blended usage (plus a small fee per account once you exceed I think 50 accounts) for all your accounts at a competitive (AFAIK close to wholesale) price.
If you have trouble contacting a suitable sales rep, PM me your email address, or PM me if you have other questions (before I start breaking the no advertising conditions).