Start by buying this
Probably the most important £30 you will ever spend. Then have a look at what is available to you locally and reference the ingredients in the book. It will give you a understanding of where the protein and energy in the feed is coming from. Always check the ME in a feed as not all 16% nuts will contain the same levels that's why some are cheap and others dear. You can feed less of some of the dear ones therefore making them cheaper. Avoid bulkers that are only in there to make up the weight!
Then have a look at the mineral levels and most importantly the delivery method eg Selplex is a much better delivery method than sodium selenite for selenium Although dearer the uptake is far higher find out what company is supplying the minerals to the feed manufacturer. Then additives like rumen buffers so bicarbonate is poorer than say alcacid. It's a minefield but use the feed directory and over time it gets a little easier.