Originally Posted by Cauldrath
Let's look at the worst case scenario. According to LoLWiki, Sona has the least health in the game at level 18 at 1600. 5% of that is 80 damage. Most mages are lucky to have a 1.0 ratio on their spells. Thus, the passive is worth at least 80 AP when used on a single spell cast. So, it effectively grants 150AP for less than 3k gold, plus CDR, health, and magic pen. Oh, and it works with spells regardless of their AP scaling - even if they don't scale with AP at all. P Blade, Sheen, BFT, Trinity Force should be a nice build for taking out squishies on a bunch of bruisers.
That's 1.) At level 18, 2.) Assuming said Sona comes into the game with 0 magic resist (impossible) and does not build any the entire game, and 3.) Assuming an AD champ built this, they'd effectively waste stats AND gold, since the only "bruiser" that would benefit from this entirely is Jax (or Poppy, if you must). And you'd be gimping Jax for stats that would obviously be worth more for a lot less (Brutalizer gives Jax more effective stats over the long run than BFT and costs less than half). And if you're building for damage and stats, well you might as well just save up 700 more gold, and buy a Hextech Gunblade for Jax and get your money's worth.
If measured, BFT is nothing but an overhyped Youmuu's Ghostblade. It, like Youmuu's, can be countered just the same.
Sorry but you must encompass all aspects of this math "algorithm" to make it work. Also, why would anyone build Trinity Force after this? If you wanted bang for your buck you'd build Lich Bane, at least the 70 AP could be added to something than just be left out there.
By the way: Force of Nature. Poops on BFT hard. If BFT does an effective 1.7 max damage per second, and FoN heals for 1.75...well there ya go