$4.50 per unit is relatively inexpensive compared to the time you would invest (and equipment dependent on what you currently own) to fabricate the blocks yourself. The pieces featured in the link you provided where not overly ornate. If you are only doing a bathroom then this would definitely seem the route to go as you shouldn't need to many.
You could fabricate these yourself if you have a band saw, table saw, miter saw, and router table set up. You may save around a dollar in materials by cutting these out yourself, however once you add your time invested you will find this probably cost a lot more then you saved. You could use the veneering method laid out by cabinetman to fabricate larger mouldings there by eliminating potential for profile differences at the outside corners. If you should choose this method or to steam/soak method make sure you calculate accordingly for spring back. However making the jigs alone for these methods will cost considerrabaly more than you will spend on the prefab pieces.
The point I am trying to make is don't fall into the trap of spending a $100 to save $5.00. It is rarely cost effective to try and produce things that are available elsewhere at reasonable costs. At the prices quoted on that site there is no way you can reasonably compete by trying to fabricate them your self.