The cheapest has to be space marines simply because they have so many units to pick from, expensive to cheap. I personally collect daemonhunters and even though i think they rule, I have to admit they are very expensive.
id say necrons, i mean if you only have warriors and a few support things it doesnt get much cheaper i eman for the same price as a tactical squad box you get 12 necron warriors 18*12 is more then 15*10 that is if you play necrons the right way (no cheese)
yea you gotta remember how much points worth of stuff you get per box so the real question is that whats the cheapest army you can get on ebay. i know tyranids and dark eldar you can get armies of those dirt cheap. just search for "40k army" or "40k lot" and see what you get.
everything except all the new stuff (tau, necrons, DH) you can get for dirt cheap.
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