Actually, how does that work?

BBB costs 12.500 gp and has 1000 pages. Meaning that it costs 12.5gp per page. So you'd need to write spells of level 8 and higher to save money using the Book of Geometry, which still makes BBB better.
It sure is, but Book of Geometry is free. When you get a Headband of Int +6, then you can start saving gold for BBB.

finishing off with either Fatespinner or Archmage levels
Forget the fatespinner, get Archmage. Prerequisites are harsh, but worth it. +1 Spell Power, Mastery of Shaping, and Spell-Like Ability (taking one of 9th lvl pays of tremendously, say Time Stop) are a must.

From my experience (and I've been playing a Batman Wizard for a year and a half now), the main problem is - the party. There're 6 of us + animal companion, and I can rarely cast area spells since we're usually fighting smaller number of enemies and once the party swarms them anything with a radius bigger then 10' becomes obsolete.

That's where Mastery of Shaping comes in, so you should get it as soon as you can.