Thankfully, Castles & Crusades: Castle Keeper's Guide has a solution for me; Buying material components costs 20 gp per level every few adventures. So a level 3 Magic-User will need to spend 60 gp on material components. To that, I'll add that if a 3rd level MU only has 20 gp on him, he can always cast spells like a 1st level MU. (In other words, if you have enough to cast like you used to with 20 gp, why wouldn't you be able now that you've become even more awesome?) That would require just a bit of note-keeping, but not that much.
That brings the obvious question though; How many adventures is 'A few adventures'? Well, the adventure Sleeping Place of the Feathered Swine has optional rules for handling differently how long torches and lanterns will last you. I won't repeat the rule here because the adventure is Pay-What-You-Want so you can grab a copy if you want to check it out, but basically you roll every turn to see if the light stays on. A MU could use that rule per gaming session (if magic was used) instead of turns to see if he needs more material components.
Yeah, let's go with that. :)
The next one about Magic-Users will deal with magic item creation in my campaign!