Oh don't ever buy online. Here is what you do.
First thing is you find a big box store, home depot type place. Right down the price for the largest bag food you can find and what companies.
Second look around for a feed store, big and small family owned. This is where you usually will find amazing prices on 50lb bags. The trick is to find them since if you don't know where they're they can be tricky.
Go here, it is the Performance Seed website and look through what they have. The best price per pound stuff is the Natures Own Signature 40lb blend, I can find it easily locally for $20 a bag. That is fifty cents a pound. And it is great premium mix of Black oil and Large Striped Sunflower Seeds, White Millet, Peanuts, Safflower, Cracked Corn. Once you locate a local dealer pay attention to what is in the bag, they have two different mixs that are not marked. One has lots of Large striped sunflower and the other has less striped and more small black oil sunflower also it seems the latter has lots of cracked corn than the former. You can't get a better mix for this price anywhere else. Call them and ask where is the local dealer to you.
If you want amazingly low prices for individual seeds, go local family owned feed stores, always buy 50lb bags and nothing smaller or the price will raise stupid high. Remember with sunflower seed it is the most used by birds and it also varies greatly in price, size, and quality. Buy a cheap 50lb of in the shell black oil sunflower for $20 and you are buying mostly shells. Buy a 50lb bag of shelled hearts and pieces sunflower for $50ish and you are getting only food with no waste and no shells, They say you get three times more food than buying the in the shell cheap stuff so the huge deal in pricing is the shelled and it is less clean up for you.
If you're buying a mix seed don't get sucked into buying "waste free" "no-mess" or "patio mix". all that stuff is..is just seed without shells at a huge huge HUGE mark up. You can find all of that separate for wayyy lower pricing at your local feed store.
If you're buying mix seed maker sure..and i mean make sure it don't have Milo, sorghum, Golden millet, red millet, flax, Canary seed and or Rapeseed. This stuff is just filler and the birds on the eastern side of the USA don't like it..you may find a cheap priced mix with this stuff but you;re mostly paying for crap filler seed and not what your local birds will eat and like.
Don't get Red Millet and White millet mixed up. White millet is awesome! A favorite food for many great songbirds local to you.
Speaking of your locals. Learn what birds you have coming to your feeder, learn what they like a little bit, more and most. Then offer the best to them, less waste and a guarantee to see them coming more often. You can easily read up online of what bird likes what once you identify them yourself. For example if you have Gold Finches then go with a Nyjer feeder. If you have Cardinals go with White Safflower with a few Striped sunflower seeds..
Don't go crazy too fast, let the birds tell you what to do in terms of what to feed and how much. I would highly recommend you going with some mix like i suggested earlier and then watch them with that and learn them better then specialize your food for them later on. Also best way to store the food is a galvanized trash can with lid, you can find these at the big box stores easily for $30 or so. Mice can't chew through it and water can't get in.
Any questions reply back..good luck!