The last few years I have been using Crop Saver from Harvest Tec and it works well.  I can't share the price as I work pretty hard on my dealer, but I also take a large quantity in 275 gallon totes.  Your crop saver should come with a rebate on it so depending on the quantity you buy you will save some money.  I have used other products on the market.  I have only found one that works as well as Crop Saver for me.  I usually buy whichever one is cheaper, but this year I pulled the trigger too early.  The other brand comes out of Michigan from a farmer/dealer who claims he is the first one to sell preservative in the US.  If you want more info on him, email me.  He can ship to you. 

There are differences in preservative.  Always check the % propionic acid like you are doing and don't be foolled by the % active ingredients.  The higher the % prop. the better.  Also, I have recently found out how important citric acid is also.  We've actually added extra citric acid to the prop to help keep the color on the hay.

Hope this helps.