The problem with that math is that you can't catch them all. Russia has been gauging Europe's response by attacking cables close by, but there's no reason to believe they won't strike other strategic targets that are harder to monitor in future.

The current approach, to zig-zag over a cable with the anchor, is extremely obvious, but they could just as well have an entire fleet of ships just casually drag their anchor when passing over their targeted cable until one of them catches it. With a bit of planning, a large number of unrelated ships could be over one or more cables at the time the cable snaps, making it rather difficult (and therefore expensive) to figure out which ship is behind the attack.

The value of their crude oil fleet is also rather diminished by sanctions now that they can't sell as much crude oil as they could before.

For this specific attack, the ship being forfeited would be a net win. However, for the many other sabotaged cables, that's not happening. I think Russia gladly pays a couple thousand euros a day to sabotage European communications for weeks, as well as trick European/NATO fleets to show themselves and demonstrate how they behave when they're looking for trouble.