One of the comments I hear from time to time from players (and sometimes DMs) about "0E" games like BX is that magic is fairly limited compared to later versions of the game. Though I've written in the past about how I see this more as a feature than a bug, the Expert rules do include a concise yet robust method to give spell-casters more flexibility and freedom. Namely the spell research and magic item creation rules on page X51. I'm going to focus on spells for now, then follow up next time with items. I've talked about both of these when discussing name level mages and their towers, but I wanted to look at the processes themselves here.
The rules for spell research are fairly loose, relying more on DM judgement and context within the game than supplying a hard and fast price list or formula. This is a better choice, in my opinion. Especially for a game like BX which eschews codifying every little detail. To quote Matt Finch, "Rulings, not rules."
The one part that is spelled out (heh, see what I did there?) is a simple cost formula for time and money: 2 weeks and 1000gp per spell level.
What this section really emphasizes is the idea that the DM and player work together to come out with the spell's level, effects, range, etc. The book describes it as a proposal submitted by the player to the DM, who then A) decides whether such a spell is even possible, then figures out the details. I think a more collaborative approach is perfectly possible however, as long as it's understood that the DM has the final say.
Oddly, when I talk to fellow BX-ers, it seems that spell research rarely comes up in their games. At least, I haven't seen many examples of it. This is too bad, since it's a great opportunity for player characters to really contribute to the game. Not only does it give their character something potentially unique about them, but it gets the player engaged in the campaign on a different level entirely.
And that, IMNSHO, is a very good thing.