You asked about hidden costs, and someone already mentioned the cost of, and addiction to, freshly squeezed orange juice. Not a hidden cost, but something to keep an eye on.
In fact I can't think of hidden costs per say, but there are things that little by little can add up.
Cafe con leche is one of them. Two to get me started at my first morning stop, anotherone mid-morning. That's 5€ gone. Ok, 3.50€ since the first one is included in my breakfast budget.
Then there are the Aquarius drinks you may order during the day to replenish your electrolytes, or even just to purchase something in a bar in order to use their toilet. If electrolytes is what you feel you may need I suggest buying effervescent tablets such as Nuun or Zym, or buy 1.5l bottles of Aquarius in grocery stores. Such a bottle will cost you the same as a can does in a bar. Yes, please, do order something in bars if you want to use their facilities: they need to pay for water, tp, cleaning staff and products.
Should you get injured, or feel you must take a taxi for some other reason, it may be useful to know that taxis typically charge 1€ per km.
I always here people mention alcohol on this forum. Red wine and beer and inexpensive in Spain (at least the cheap stuff) but again that adds up if you drink beyonf the red wine included in the menu del dia or pilgrim dinner. And then you may also be tempted to try local specialties: cidra, pacharan, orujo...
Meds on the other hand are a bargain, and many that require a prescription at home are sold without in Spain, but behind the counter. Ah, but farmacias charge a bundle for suntan lotion, bring it from home, at least a bit in case of a super hot and sunny day.
And the other unexpected expense maybe bag transportation, again in case of injury or a day you feel you really could not walk another day with your pack. Typical price is about 5€ these days, perhaps as high as as 6€ but less expensive starting in Sarria. This also adds up.
Finally laundry if done by machine. Depending on the albergue you could pay from 2-4 or so per machine (washer and drier) but prices vary a lot. Sharing a load with others is a way to save. I use those services after a rainy day when my pants are covered in mud, or I will pay for drying only on rainy evenings when I know there is no way my clothes will air-dry. Laundry powder is now most often included, even dispensed automatically from the washing machine.
Perhaps others will think of ways € manage to quietly escape one's wallet....