This isn't a great idea.

Here's why: what you'll easily notice more than losing 100g in the BB is a stiffer 2-piece crankset like a Hollowtech II or something, and titanium is flexier than steel. You can find such cranksets second hand these days which means you find something much stiffer and lighter within your budget if you ditch the square taper cranks.

And anyway, it's one of the least noticeable places on the bike to save weight. After tyres and rims, it's the seat, then bars/stem/seatpost, on down the bike.