The other thing to keep in mind is that a solution may require multiple techniques for it to be effective. This can mean a higher upfront cost to the client, but then a better return on investment down the road.
It may also be helpful to think of this situation as a different animal/conflict. A good example I believe would be pigeons.
If you were dealing with moderate to high pressure with pigeons feeding/loafing in a building but nesting and night roosting on the neighbor's property, how would you handle it? Choices may include netting the structure or installing other types of exclusion products, but you'll have to include some sort of population reduction program to get the maximum effectiveness.
Getting back to the groundhogs, based on if you want to relocate the animals or have to use lethal control, I'd look at doing at least a 5 day trapping and/or shooting program to remove the resident 'hogs that have an affinity to the site location.Then I'd try my exclusion devices/products and/or install the exclusion wire. Also, you can see about digging out and repairing the tunnel damage or having them filled with concrete backwash to add even more security to the site. I'd also do a maintenance contract with the client so you can monitor the project area and take action quickly if necessary.
By combining multiple techniques with this situation, it will give you the best chance of having a cost effective, long-lasting solution for them.