The locking mechanic is one of the most important things to understand before rolling Irisalis Transmog.
When you find an appearance piece you like, locking it allows you to keep that part while continuing to roll for others. The problem is that additional locked pieces increase the cost of the next roll. Community testing indicates that the cost scales upward as more parts are locked.
Because of that, randomly rolling without a plan can burn through currency quickly.
Read this Irisalis Transmog lock cost guide before trying to build a specific look.