at my co-op, it's $5 for a 5 gallon bucket of whatever of the 3 fertilizers they have. here are the bulk prices per ton from the
www.sekgrain.com website.
46-0-0 $248.14
18-46-0 $256.12
0-0-60 $153.52
you can rent a pto driven spreader for $6/ton, it holds about 15 tons of material.
i think you are paying extra because it is sacked, and it costs money to sack it. if i bought 500 lbs of 0 0 60, it would be $38. 50 lbs would be $3.80
the bulk buying savings on the P isn't as much. 500 lbs would be $64, 50 lbs would be $6.40 and that's not too far off from the $8 you pay. i think the extra money sort of has a convenience factor in the handling, transportation, and measuring of the product.
but in the big picture, if you bought it in bulk it would probably cost you $275~$300, but then you have the pain in the azz factor of transporting it since it's loose.
spend the $350 and start the work. this fall, the extra $50 bucks or so won't really matter.