If sort of feels like maglev should be able recover a decent amount of electricity during braking

Almost certainly as much as regenerative breaking on an EV, so upwards of 90%. Like you mentioned, it's the same thing in another shape.

A high speed train probably doesn't break all that much, though, and I'm guessing wind resistance is the biggest energy cost either way. Which is why people talk about putting a maglev in an evacuated tunnel, although you now have an oversize oil pipeline around your maglev tracks adding further to the cost.

Maybe it comes down to just a handful of magnets round a few axles being cheaper than a long line of magnets the length of the track.

It's true. You can make a pretty cheap magnet, but never as cheap as a couple stupid rods on the ground (although at high speeds I bet it's not just rods but ultra-precise rods). Some systems put the magnets just in the car instead, though, and usually with high-speed rail acquiring a continuous corridor of land to build is the big hurdle, so it might make sense to go as high-end as possible once you have it.