The magistrates are able to charge several hundreds of pounds costs, and are quite likely
to do so if they don't think the matter should have ever come near them, and that the
defendant can afford it (IANAL)

Nowhere near right,I'm afraid. Costs are in the discretion of the bench, but for a simple
summary offence on a guilty plea are unlikely to be more than about £60.. Costs are not
used to punish anyone. If you plead not guilty and papers have to be prepared, counsel
briefed, and witnesses brought in, then of course costs will be higher. If you are
acquitted, there won't be any costs at all and you get your own refunded.