You'd have to look up the relevant laws in the country concerned, which could require some research and quite possibly translation. For example, in the case of the US, I believe it's up to $4,300—now $5,340—but there are a bunch of programs in place that can result in the fines being waived for airlines under some circumstances. This document lists a broad average of $3,500/passenger, but notes they can go as high as €10,000. The airline can often try to have the fine waived or lowered if they showed that they took reasonable efforts to verify the required documents.
Note that the fine is generally imposed if the airline has failed to adequately verify your travel documents, rather than if you are just denied entry in general for reasons entirely outside of the airline's knowledge or control. The airline may be responsible for your return transportation in any case, costs they may seek to recover from you and/or any outstanding funds from your air ticket (e.g. if you purchased a round trip ticket).
But an indemnity form is somewhat beside the point for a few reasons:
- You probably already agreed to indemnify the airline for immigration fines as a result of your travel. That's a common provision in airline contracts of carriage.
- Such forms are quite uncommon and not usually an option. While they do exist, at least in the case of one airline (Qatar), the purpose of travel document requirements is to prevent unauthorized travelers from boarding a plane in the first place, not to collect fines. If everyone could just sign a form to be allowed to board without the proper documents, visa regimes would be rendered fairly useless. Countries do not want someone who, say, plans to apply for asylum, to be able to fly to their country without a visa simply because they've signed a form. It's possible some airlines would let you sign such a form under some unusual circumstances where your documents are present but questionable, but the most likely scenario is to be denied boarding if they don't accept your travel documents.
- The airline has no real idea of your ability to pay the fine and limited means to collect it. If a fine is more than a theoretical possibility, they may rather deny boarding than ask you to promise to pay a sum you might not even have.
It's possible that there could be other costs as well: a return ticket, detention fees, a fine that you owe the country directly, etc...