Travel nurse contract salary changed at last minute!

That definitely doesn't sound like a mere typographical error but rather a significant change in the terms of the contract.

However, it's not entirely implausible that it was an error as a per diem rate of $357 for "meals & incidentals" seems very high. A reasonable comparison is the per diem M&IE rate for federal employees which varies by location but ranges from $46 to $71.

I'm not a lawyer and no one can offer professional legal advice on this forum so you should know that any advice you receive here shouldn't be substituted for professional legal advice. You may want to have an attorney review the original contract and advise you on what rights you have to enforce it.

If the original contract was signed by someone with the legal authority to execute such contracts on behalf of the company, then it is likely that it is still valid and enforceable. However, you should read it carefully as I would be surprised if it didn't contain a number of clauses that gave them wide latitude to do almost anything. Since you probably only had an opportunity to negotiate terms like the pay rates and not the actual contract text, it's very likely the contract has numerous clauses that strongly favor their interests rather than yours. If you refuse to sign the new contract, they might decide to just find someone else for the assignment. Even if you have a valid claim against them for breach of contract, litigating it would take a long time (could be months or even years) and cost a lot more than your maximum potential damages at this point.

However, you may still be in a good position to negotiate as at this point they've likely already spent quite a bit of money to find and recruit you and they would probably rather not lose you. I suggest you point out to your recruiter that the new contract is unacceptable because it reduces your pay far below what the company already agreed to pay and the minimum amount that you require so you are not losing money by taking the assignment. If you can't work out an acceptable new deal or get them to honor the original one, then you may be stuck with no job or a new job paying less than you expected unless you can get your old job back somehow.