The costs for Droid Locomotion are in Table 11-3 on p. 188. They reference Cost Factor, which is in Table 11-2 on the page before and varies by size.
Per p. 189
Jump servos double the cost of a walking locomotion system.
The rules make no distinction about when a property is applied, either as part of a new design or as a modification later, so there's some GM discretion involved. You have a few choices...
Subtract the cost of the droid's "free" movement system from the doubled cost of the enhanced one. In effect, charge the price (Table 11-3) for whatever the base movement normally costs. "Half" of the doubled cost is free, the other half you need to pay for.
Charge the full cost (Table 11-3, then doubled). This reflects that the droid may have been built one way, but was later modified to include a different capability.
The base movement is free, and zero doubled is still zero.
I've listed the options from most-to-least fair; #2 is stingy, and #3 is twinky. No matter your decision, take into account the existence of Jedi and Force-enhanced movement. I would only take option #2 for an after-character-creation change, when money is a bit more available and it's logical for the droid character to need to replace or completely rebuild his legs to get the added capability.
Note: There's a "Military Engineer" prestige class in the Clone Wars book. I don't own it, so I'm not sure if that book may offer additional guidance.