It's pretty shallow[0], looks like about 10m deep around the abutment that was hit; I would think that a big pile of rocks around the abutments that supported the piers on each side of the channel would do it. I just found out you call this RipRap[1] in the US which is cool, in Britain it's called Rock Armour. Total guess but a hundred meters in either direction parallel to the channel by 50 wide? That's about 100,000 tonnes of rock. Something like that? Something close to $50-$200 / tonne delivered? So range of $10m-$40m for both piers, so call it less than $100m. These are guesses which I am making from another continent! so probably the right order of magnitude but not much more accurate than that. I imagine it would have cost a lot less than $8bn dollars, 6 peoples lives and the indirect disruption to the US economy. With hindsight a suspension bridge or a cable stayed bridge with piers well away from the channel might have been more suitable. That style of structure is pretty old-school even for the 1970's, I wonder if it was designed like that because of ground conditions or something, or that was just the cheapest way of doing it in the US at that time and place?

[0] https://webapp.navionics.com/?lang=en#boating@15&key=gqjnFz%... [1] https://en.wikipedia.org/wiki/Abutment

Edit: Changed distribution to disruption.