As mentioned, use cheap mono as backing and spool up with 50 yards of braid. I also use a mono leader, but that is more personal preference. Braid is a pain to tie, and the mono holds up better to abrasion.
I prefer 10/1 braid, smaller diameter, get more depth with less line out. I have used Fireline, Power Pro, Spiderwire, it all works about the same for me. Fireline is a little easier to cut and tie, since it is fused.
I spool my 15-sized reels with 920 feet of 10lb Trilene XT that I've had on a bulk spool for 15 years and top it off with 300 feet of 12lb Big Game in the high-viz solar color. Every year I take off the topshot and cut at the double uni knot used to join the two together. I've had the same backing/filler mono for six years, it's just there to take up empty spool space in crappie trolling.