Simon, the prices for Authorize.net vary with the reseller. You'll find the resellers in the reseller directory. The price structure of Authorize.net is different from VeriSign in that it does not depend on the number of transactions/month.
To give you an idea of the cost, here is what I paid through Cutting Edge:
- $195 setup fee
- $10.00/month online host fee
- 2.15% discount rate for MasterCard & VISA transactions
- $0.30 transaction fee
It pays to try various resellers as fees are negotiable, especially the recurring fees.