In any standard economy, the price for an item is dictated by the fixed cost, variable costs, and expected profit margin. Translating this into your campaign is relatively simple but may require some additional planning and record keeping.
Creating a magic item requires:
Shop space to craft the items-one can probably assume that powerful magic items are not being created in the corner of the local tavern, or in the middle of the public street. There must be someplace where the atmosphere is appropriate for the concentration and casting of spells can be done for the 8 straight hours to complete such work. Now, a "professional item creator" probably has a permanent arrangement for this space, just like any other profession. Whereas an adventurer would probably have to rent or make such a space just the same as a character wishing to manufacture any mundane item would. While the details of such an arrangement can be left out of the campaign (he is renting the Joe the Scrollmaker's backroom for a few weeks at x silver per day), the additional cost should not be ignored. Or at least not without impact. Imagine a mechanic is travelling, yeah he will have tools for basic repairs on his car, but he is likely unable to completely rebuild his engine without first finding a functional garage.
Variable costs - these would include supplies, tools, beakers and glassware... a local vendor would have local supplies for these things, whether it is knowing where to gather the purple moon fungus, or even have a supply of tools that the character would not readily be able to carry. Imagine carrying a full chemistry lab in a backpack. These are costs that a character would have to pay, but the professional would find at a discount.
This leads finally to profit margin, the local item creator would find that he could earn a living selling large volumes to regular customers, whereas a character might just break even.
This is similar in nature to asking why anyone with a skill set to build something in the real world can't just compete with large companies building the same exact thing. Can you build a computer as cheaply as Dell or Apple? Of course not, they have vendors and machines in place to build those items. Now try going to a city where you don't know anyone and build them on the street corner. Or how about manufacturing a car for the same price as Ford or Chevy?
Now the flip side of this coin is, if you have a PC that wants to devote a significant amount of time and effort becoming a professional item creator, and there is room in the campaign for this activity, I don't see why temporarily unoccupied characters would not be able to get a job using any of the skills they possess up to and including item creation. Though, for simplicity sake I would assume that they were working for someone else rather than setting up a business for a variety of reasons. If as a GM, I were asked why, I would probably reference a law requiring acquisition of a business license. My real reason would be the insane amount of bookkeeping required would bog the game down.
Next let's hit on the difference between salary and contractors mentioned elsewhere in this post. While it may cost 50gp per level to have a contract spell caster cast a spell for you, this does not assume that said spell caster receives that for a days work. If you walk into any contract service company, they will charge you a certain rate, at best the person actually performing the work might get half. The rest goes towards business overhead, supplies, fees, licensing, ect.
There is no reason to believe that the local mage guild is in any way different and don't get me started on the expenses of organized religion. The Catholic Church is one of the largest financial organizations world wide.
If the characters are seeking temporary employment within your campaign, (which I might add makes perfect sense in a realistic campaign, as an exsoldier I can tell you that it is always hurry up and wait), I would figure out the daily wage for any other highly skilled profession, then adjust for the relative scarcity of magic in the campaign.
Sorry for the long answer hope it helps.