Probably, the most expensive .357 magnum ammo
would be the old armour piercing round that
Winchester-Western use to produce. And, beside's,
its illegal for civilians to own. Under the ole'
theory, "this round would bust the block in
automobile's".
My next guess would be the "Super Vel" 110
grain JHP, as manufactured by the late Lee
Jurras. His advertised velocities approached
1500 fps.
Best Wishes,
Ala Dan, Life Member N.R.A.