What option is cheapest will depend on volume of transaction value you expect to process. May also depend on how risky the transfers are.
For very low transaction volume your costs to set up the integration in first place may dominate the total cost, not the per transaction fee.
If you have high transaction volume you could shop around different payment providers for customised quotes.
For sufficiently high transaction volume it might be cheaper for your business to internalise some or all of the work to process the payments, but at that point you would need to be setting up systems to solve many of the problems that payment processors deal with.