Forum OpenACS Q&A: Ballpark estimates, they plump when you implement them

Custom programming the integration for a given payment gateway (Verisign) probably has taken about 3 developer weeks to get a pretty robust system in order to communicate with the gateway, and another many scattered hours of non-dev time are required to make sure the bank and the processor and the gateway are all talking to each other.

Probably would be a bit easier to adapt the payment gateway service contract, but I haven't gotten to look at that yet, so I can't speak to that too well.

Feel free to take a look at our General Billing and Verisign Interface packages, as well as the Event Registration package which uses them. Mind you, these are all set up for MACS, so you'd have to put in some work to apply them to OACS.