It's fine as is. AP rounds aren't that expensive considering that they're a significant upgrade. Make it too cheap and you can use it every single time a tank shows up (Like incendiary rounds on mg42), at 30 it's just expensive enough that you have to use it tactically and not waste it.
AP shells for the 57mm doesn't have the impact Incen AP does though - it's not like going from damaging to possibly one-bursting light vehicles, it makes the 57mm go from probably not damaging tanks to damaging tanks in a fairly average manner (assuming the slightly lower pen but high fire-rate balances out). The cooldown is significant enough for the decision-making involved in it, and a player should not be hampered like that to use AT guns for, well, AT.
When I think about it some more, I'd think it entirely possible that it'd be necessary to tone down its other advantages in exchange for that. But I really think the cost of the 57mm's AP shells just ain't right, it'd be a bit like if a mortar had to pay munitions each time to shoot over objects.