As DJClayworth says in his comment, does your brother have anything in writing saying this is the cost of the policy? Even a printout of the web page where he bought the policy. Best if he has a signed contract.
In general, if someone signs a contract agreeing to provide a product or service for a specified amount, declaring that you made a mistake when you signed the contract does not void it. If it did, contracts would be worthless. Anyone could sign a contract, see how things work out, and then if they decide they're not making enough money, just declare they made a mistake and the contract is no longer valid. What if he went back to the insurance company and said, "Oh, when I said I could afford to pay $500 a month (or whatever number), I made a mistake, and I can really only afford $400. So that's all I'm going to pay but I expect you to still provide the same service that you agreed to for $500" ... do you think the company would just say, "oh, okay, if you made a mistake ..."? Of course not. They'd tell him that he signed the contract and he is now committed. That's what contracts are for. A contract is a legal binding commitment, not a statement of what you think you might be willing to do if things work out.
If I was your brother, I'd get together whatever paperwork I can find saying that this is the price, and then tell them that if they do not live up to the terms of the contract, he will be contacting a lawyer to investigate suing them for breech of contract, and/or contacting law enforcement to make a fraud complaint. If they persist, I would contact a lawyer.
I'm assuming here that they really did advertise that they would provide this policy for the stated amount in writing or on a web site. If they said the price was $1000 a month and he's only been paying $800 a month and they've let it go but now they say the want the difference, different story. Or if the web site says it's $1000 a month but someone told him on the phone that he qualified for some special deal for only $800 a month, well, verbal contracts are theoretically just as binding as written contracts, but it's very difficult to prove what was promised.