Feedmark Benevit Advance - once you buy the first tub (5kg) you can buy refill bags saving money that way. Delivered next day without fail and they often have special offers.
I've fed 365 Complete and found it pretty good for money value and keeping my horse looking great, very shiny. I've also fed Feedmark Benevit Advance, again I like it, my horses look great on it but I had to half the amount it said to feed them as I found it heated them up a little.
I fed Topspec Lite balancer for while but didn't like that as much - didn't notice any visible effects and found it expensive
Codlivine is reasonably priced and people have said their horses look well on it (could just be from the oil in it making their coats shiny though!). Some people aren't keen on the idea of feeding horses fish oils though. They do one with an in-built joint supplement as well which I think is a nice idea.
It's a bit like buying your vitamins from poundland versus a premium supplier. The cheaper versions may well be much less effective and less well tested and controlled (synthetic versions of the vitamin for example).
Given that a broad spectrum vits and mins supplement is on general principals not based on a known specific deficiency, I'd much rather suggest a higher quality range (top spec or science supplements) but given at a lower dose so that it works out the same price . At least what you are giving them is then proven and going to be absorbed efficiently.
Progressive Earth do one that works out about a tenner a month. Its not as high specification as the others they do but is designed to be a decent broad spec balancer for average UK grazing/forage