You can already add early grav lifters at TL8 using the prototype rules, but they're big and power hungry, and orbital range only.
If you add the orbital range double disadvantage, on top of the intrinsic prototype disadvantage, you're looking at 600% cost, -50%. Since I can't find the rule I once assumed of 3 disadvantages max, you could boost it up to 4 disadvantages*, and get it to 600% -75%.... Which works out to be +50% cost on top of a normal M-Drive-1. On something like a free trader, you'd have a 2.5 ton drive, needing 26PP, and costing MCr6. Best paired with a 1g reaction drive...
*Not saving anything for the intrinsic prototype disadvantage.