you can't buy rares, thus there's no REAL buying price and I could count their price as 0, their cost is subjectively 0 as you don't pay for them(maybe the match price in case of reg rares, but 0 or even negative values if you consider nymphs usually come with money from the oracle).
The prices in xeno calc and in wiki are just speculative based in patterns from other card's price, and if they're going to be used as the rule then it should be clarified in the OP