It would be best to discuss this with your account team, they can help you with the budget and part numbers questions.
From sofware compatibility matrix it shows you can indeed not directly upgrade from 7.0(2) to 8.5(1). Though upgrades within the same major release can be done with the downloads from CCO. So you can do an intermetiate step to 7.1(3) or 7.1(5) release. From there you can do a direct upgrade to 8.5(1) though for that you would indeed need to purchase the upgrade (as it is a major upgrade from 7.x to 8.x)