I agree with most of what Feelnrite says about hay.  Grazing my own ground works and buying my winter hay works out for me. However I do think the answer to purchasing hay/feed will vary widely depending on how much ground one has, where it is and how much time you have. In some areas there may not be a consistent supply of good local hay to purchase.

I don't buy "feed", other than gentling treat grain, just hay. 

Buying hay is like buying land in a way.  Making good hay requires land, fertilizer, equipment and most important, timing.  I am short on all 4 of those.

Purchasing all hay can work. I suggest that you establish a long term relationship with a good local hay producer.  It helps him to know how many bales of what quality (dairy or beef, protein/energy or filler, etc) you will need for the year and gives him a base of business while it gives you a reliable source of supply.

I do not think this is an area where you want to always go with the low bidder. You may not have to pay peak/market prices but good hay at a price fair to a reliable long term producer is not going to be cheap hay.

It seems to me that some folks that make their own hay don't have a good picture of the total costs involved to make good hay.

There is no one answer to the OP question but buying all hay can work in some areas of the country.  

Jim