I hope that $90 per 50-55lb sack, which is a bit expensive but not SUPER unreasonable. If they're charging $90 for 5lbs that shop would never get a penny of my business.
I can't answer you specifically but the cheapest option is almost always an organized group buy. Group of homebrewers piggy back on a commercial brewery order. You'll typically have to come up with 2200lbs of grain (a full pallet) split between you but allows you to purchase at wholesale prices (think 40c per pound instead of $1.90).
I believe I paid 76 at Niagara Traditions in Tonawanda @Ragman. It was a barke pilsner 50 pound sack. Also, the beer club I'm in is afforded opportunity to bulk buy every once in a while. I gave up on decent LHBS shops around Rottenchester! I don't think any new ones have showed up.
The owner of my local brewpub will sell me base grains at his cost (about $35/50lb). I know some States have grain tracking laws that wouldn't allow that. It might also be different if we had a LHBS, as he might not want to undercut their business, but the nearest is an hour away.
Another trick is to break up the bag. For instance, if you order a 55lb sack of 2-row from Morebeer, it will cost at least $87 after shipping. However, if you order six 10-lb bags you pay a little more per pound but get free shipping so 60lb only costs $72.