it is good idea but I do not wish to make it completely free (and I can not too). any idea of how then make money?
you can get fee for your services, essentially any transaction processed from your gateway will take some fees (hence the services fee). if you add another fees and your client needs to get registered then i doubt it will look good, as other payment gateway would probably just use service fees.
as others have mentioned, there's also a free service for this purpose so you should think of other features that want to make your clients choose yours instead of using those free services.