I understand that Gravity Forms are not a complete ecommerce package, and as it stands covers most of our requirements for building forms for customers buying/enquiring on a number of services across several business units that we provide.
Gravity Forms currently does price calculations as; Product + Options * Quantity + Shipping = Total
However, it appears that Gravity Forms has no real concept between a reoccurring cost and once-off fees. The desired goal is to segregate the pricing into reoccurring and once off charges. This doesn't need to work against existing payment gateways that Gravity Forms supports and does not require determining the actual price from a given reoccurring period and base per period pricing.
How difficult would it be to write a plugin or alter gravity forms for pricing to function as such;
Reoccurring charges; Reoccurring Product + Options * Quantity + Reoccurring Shipping = Reoccurring Total
Once-off charges; Once-off Product + Options * Quantity + Shipping = Once-off Total
Making changes to Gravity Forms itself is something that would preferred to be avoided due to complications in upgrading and support.