I drill about 75 to 90 lbs per acre. 75 is about a bag and half depending on seed size.
We're not farming for production so I would think 4 bags on 2.5 acres would give you a decent outcome.
I've never broadcasted them but read that guys who do tend to seed heavy so I guess you could go 5 bags. If it were me I'd save the $60 for gly.
quick answer 3 bags, maybe 4. 140,000 seeds per unit, one unit bags. Soybean seed size varies all over the place. Ag beans can run roughly 45 - 58 lbs per 140,000 unit bag.
If you set your disk harrow shallow, 2" into soil, and make one pass ... you can bury broadcast soybeans an average of 1". This works best with tow type disk harrows, and harrows with smooth edged disks. The more seed in the ground, the less seed gets eaten by birds and critters.
I have had great luck broadcasting mine on top of the ground then going over them with the tiller. One pass is all that is needed to till and bury them. Sure some get to deep and some to shallow but it has worked for me and I will continue to do it that way.
Before I had a planter I broadcasted many times with beans. Disc, spread one bag per acre, cultipack, done and it worked great. There were times I wish I had gone heavier on the seed though. 3 bags for 2.5 acres should be good.