[Cyclamen]

それ is referring to なにか, that "something" that is going to be "obtained (得る)" .
と is a particle used as a connector, in this case linking the two terms which are compared (that have the same value). In this case, the first term is the thing that is going to be obtained, the second one is the thing with the same price (同等 の代価のもの).

I'd translate that sentence
"The general rule of fair exchange explains that, in order to obtain something, is necessary something else with the same price"