This is silly. Not only are prices and all over the map for, well, everything... acid, RCs, shrooms... but for LSD anyway potency can be very low or very high. And the effects are so all over the map as well, it is just a totally meaningless question purely on these technicalities.
But in a deeper view, cost is irrelevant (as long as we're not talking some sort of absurdly rare and extremely expensive oddity) when dealing with seeking an experience that so profoundly alters your mindstate and that is only taken rarely. Well I guess some may be in the habit of tripping on SOMETHING every weekend if they can (as I did for a few periods in my past), but that gets boring pretty fast, and even if you do that, how significant really is the distinction between your weekly dose of whatever costing $5, $10 or $30? OK maybe $100/month, hardly seems like something you would base your decision on... who in their right mind would/should choose something that gives them an inferior experience just to save a couple dollars? That's what mystifies me about the original question, I just can't understand anyone making that tradeoff. (Sorry fly, nothing personal). I guess if you are buying in large bulk quantity to sell for profit, or acquire a lifetime supply of or something, but still, it seems severely short sighted to be worrying about pocket change when you are talking about a "Key" that unlocks The Mind! It's about the experience, not the price per dose.
But having said all that, the answer is obvious (as long as you have a good connection that is making strong hits at a fair price): LSD.