First the question. Christian Kolbe asked:
Are the gp spent on mana balls also converted to XP? It is just a different way to look at investment/spending of XP. You might even reduce the XP required to level since they have to invest XP in each spell.I gather here that you are using an "XP for gold spent" rule. That's a house rule that has been floating around on the net for a while, although I'm not sure how common it is. I've never ran or played in a game with that rule, myself. It's always been "XP for gold brought back from an adventure" for me. If you use that rule, or any house rule that does away with XP entirely, you aren't investing XP in any spells, so changes to the XP required to level up wouldn't be appropriate.
For GMs using "XP for gold spent", however, it depends on what counts as "spending gold". If it doesn't matter what the gold is spent on, then paying for ingredients to make spell balls gets you XP, the same as paying for anything else would. You wouldn't want to reduce the XP required for an M-U's next level, in that case, because the M-U isn't being penalized for prepping spells instead of spending cash on booze.
But if I recall correctly, Dave Arneson's original pre-D&D rule was that characters had to spend their treasure on non-adventuring items: hobbies, property and property upgrades, social events, anything other than gear you bring into the dungeon. Several of the house rules I've seen for "XP for gold spent" go this route, such as the various carousing tables. In those cases, paying to prep your spells would reduce your potential XP and theoretical slow your advancement, so it might be better to adjust XP requirements for levels.
Fortunately, I made the cost per spell ball 100 gp, which would be 100 xp, and if you only spend one day prepping spells before an adventure, the xp invested would be 100 * your level per adventure. You'd have to gauge how many adventures it takes to get to the next level, and that is going to vary from GM to GM, so maybe a better way would be to give magicians an XP bonus per spell prepped.
... Or just make an exception and allow gold spent on spells to count towards XP. That's probably smarter, since the rules as I wrote them allows magicians to overstock on spell balls or avoid paying for ingredients entirely. If magicians get XP for prepping spells, a player could opt to not spend any money at all, but just gather ingredients and make spell balls all year instead of going on adventures.
That leads in to some other things I wanted to say about spell balls, but I'll save that for another post.