What expenses is it absolutely impossible not to pay in order to defend against a lawsuit?
I’ll defer to a civil lawyer, but off the top of my head, if you decide to represent yourself, and you are poor, you may file a motion to proceed in forma pauperis, which means the court will waive your filing fees. So in that extreme case, you could, I think, theoretically avoid any costs.
If you’re not at or near the poverty line, then you have a certain minimal filing charges to file your answer to the complaint.
If you’re a corporation, then you can’t defend yourself and would have to pay a lawyer.
It probably varies from place to place, and perhaps from case to case, but in general about how much are these charges?
And if you refuse to pay (and so don’t get to file your answer) is the case automatically decided against you?
in New York trial level courts there is no filing fee for the (non corporate) defendant to answer. There is a filing fee for the plaintiff only (technically, you are paying the fee to receive a docket number, not for filing your claim, but in the end they are the same thing), and filing fees required for certain other actions, most of which only apply to the plaintiff, or are normally sought primarily by the plaintiff. The fee for a motion is $45, and a Demand for Jury Trial is $65. As noted if you truly cannot pay you will be able to proceed on a waiver of such fees.
If you were properly served and make the conscious decision not to answer you will be held in default and lose. Under limited circumstances, you can get the case reconsidered. “I didn’t feel like answering” and “no one can tell ME to pay some dumb fee!” are probably not one of those circumstances.