There are a plethora of payment gateways in the market and we can't integrate with them all, not on our dime at least. We're currently integrated with Authorize.Net, BluePay and eSelect Merchant services and would be happy to entertain integration with another gateway, but we'd need to pass the cost along to you, the customer.
That said, it is hard for us to anticipate what the cost might be. Each gateway is different and requires a different level of diligence from us. The hard part from our side is already done in that we are PCI-compliant and we have a shopping cart built and in use with other gateways, but the cost will probably range from $1,000 - $3,000. Also, it might take as much as 4 months for us to fully implement and test. We would want to arrange this as a Time and Materials engagement where you pay us for whatever work we perform, so if it takes 10 hours we'll bill you $1,000 or if it take 30 hours we will bill you $3,000.