Some old school luminaries have been blogging about spell copying. They wrangle over cost and time vs level schemes. But since my games are about adventure and not bookkeeping or resource management I'm thinking of doing something different. I'm going to hand wave it. No cost, no time. It happens "off screen" from the main action. There are however a few provisos. Spell books still cost money. The magic-user needs a place of study to do it. And the materials like pen and ink plus paper. Minimal cost but they must be available. Also in my world spells are secret and rare. So before any spell caster thinks about tossing off a wad of scrolls they should think about one thing. Every scroll is another chance that someone , perhaps and enemy could acquire one of your spells. And if they find a extra spell book they can know all of your spells. I like to run a Vancian style world when it comes to magic. Jealous mages guarding secret tomes of lost knowledge fragments. No magic-user knows more than a few of all the spells known. Anyone who can recover all the know spells in the world would be very powerful indeed. Motivation for adventuring. Now potions, thats a different ball of wax or kettle of fish.
Rule for Learning A Spell ; The DM rolls a D 20 vs the magic-user's Intellegence score if the roll is equal to or lower the magic user can learn the spell