Hmmm, ideally this should have been part of the original discussion and contracts with the vendor. A no. of factors would impact the cost, e.g. support hrs (9 to 5 or 24x7?), and response times. Is the software totally unique or just a variation on what they've sold to other customers? Cost for support & changes may also be different, depending on level & complexity of changes. From a commercial point of view, what's it going to cost them to provide the service & what can you afford to pay for it. Ballpark for a maintenance contract, I'd be looking at 10% pa.