The miners only reach ROI if you are very optimistic about future events. Price will not go below 100, difficulty increase won't speed up further, and most importantly, the miners will be up and running in a month.

What I don't see is why you dismiss the option of refunding chip costs and the shipping fees. It's not like it costs you anything, you're not refunding the components you bought, you're not refunding any surplus you might've gotten for the chips. If there are people who want their miners despite the not exactly stellar odds, and it seems that there are, you can still do what others did and only ask for refund for one of the two orders, or combine orders with other group buys.
Really, the very least you could do is ask everyone what they want and act accordingly, since you're managing a group buy that's kind of what you should do as well.