Different varieties come in different size boxes. Cavendish and Burro come in 40lbs cartons, Manzano and Reds come in 20lbs cartons and baby bananas come in 15 lbs cartons. The Los Angeles terminal market carries all these varieties and has price updates online daily.
LA terminal Market Apparently Nam wah bananas are packed in 30lbs boxes but their movement is not great enough to register on the market report. From what I have gathered retail markup on most produce is about 30%-40%. From what I have recently seen in grocery stores it may be even more as bananas at my local grocer are $0.69/lb