history - Kashrut with very costy salt?

Currently the cost of the salt (for human consumption) is practically negligible, mostly around $1/kg.

But it wasn't always so. There were centuries as salt mining and selling was a governmental monopoly, and the government used it as some type of a "luxury tax" - similarly, as we can today see in the case of the benzine in some countries. This made salt prices irrationally high.

But making kosher meat requires a lot of salt.

What should be done, if a kg of salt would cost a hundred dollars?

Answer

First of all, meat can be eaten raw (unsalted, unroasted, un-anything, straight from the carcass) after just rinsing it (YD 67:2).

Regarding salting, a non-trivial number of rabbinic authorities (even current ones) have allowed using sugar to 'salt' meat when salt was not an option (for availability or medical reasons). See this article for a sampling of sources.

There are two other ways though of permitting meat to be cooked which do not require salt: roasting (YD 76) and blanching (YD 67:6, 73:3). While roasting is still employed nowadays by all even lechatchila, many communities nowadays are accustomed to not blanch unsalted meat, though bedieved or in certain pressing circumstances even some in those communities permit its use.

You should certainly speak with your rabbi before attempting any of these methods or if salt poses a health or financial risk to you.