It depends on who I am dealing with and also the drugs, I take a wide variety of drugs and have had many sources for each over the years. Back when I would take pills there were a few times with a couple dealers I had where I was able to return the majority of pills I had bought minus the ones I sampled to find out they were shit, on all occassions I got a full refund less the cost of the pills I had consumed.
A few times mates have sourced bunk pills or gear for me and when it turned out to be bunk didn't expect me to pay them for it, but I would of been willing to had they not insisted otherwise because they weren't profitting and it isn't really their fault.
With weed I have on occassion been discounted on my next purchase after complaining about the quality of the last batch. There are also quite a number of occassions I refused to purchase weed I had agreed to buy after seeing it and assessing it as lower than acceptable quality.
Nobody I buy ice off these days would ever give me a refund, EVER, they wouldn't sell me bunk shit but there have been numerous occassions that quality was sub par but if I called and complained I would get laughed at, that is just the game really. The truth is they are stuck with the same gear you are. The same would be the case with anywhere I have purchased heroin. With ice and speed there have been one or two people after supplying a sub standard product several times in a row and it isn't like I didn't tell the bastards exactly what I thought of their shitty product, but I didn't waste time asking for a refund.
I didn't vote in the poll because I don't think there is a black and white answer. The relationship you have with the supplier, the drugs you are buying and the quantity you are buying all have a bearing on it.