It sounds like there are two things: 1) a fence between your property and the common property. It might be reasonable for you and the OC as a whole to share the costs for that. 2) A separate issue seems to be a fence around the common property which would be a cost for the entire OC and consequently your contribution would be your fraction of the unit entitlements.
In our OC unit owners are permitted to have fences around their unit areas but it is not encouraged. In our case the unit owner pays for and maintains a fence on their unit boundary if they want to have one. We also have a fence along part of the overall property boundary. An allocation for occasional maintenance of that is included in our sinking fund plan.