Have you transmuted anything onto your Arrendal yet? Give it a try. I did. Pres stones cost $16 USD each in RPs! I blew $64 USD with 4 failures in a row befroe I said F**k it and left it with the stupid 1% leech you get in the beginning. I sure as hell will not be trying to transmute any of my upgrades onto it.
They are expensive because you can't use any of the armor from your other classes. I have a lovely Dex Onyx set that I would love to use, but I can't simply because the Trinitys can't use armor that any other class can use.