well then, that's no fun. I better stick to my job before i call the boss. But if the S7 drops value that fast, shouldn't you be able to buy an S3 "buy 1 get 10 free" style?
Values of miners eventually kind of bottoms out. New miners always devalue very quickly.
The S7 was 1800-1900 at first and now its 1000-1350 in very little time. Eventually it will stabilize, since soon after a new generation enter, the difficulty goes mad. Value of hardware depend on profitability, so with new hardware entering the market automatically = to a devaluation of every miners.
The S3 are very nice and pretty cheap, you can probably get some for 80$USD~ But its useful for others, while probably not useful for you, because of your electricity cost.
However, if you need heating during the winter, then buying some miners to heat with ASIC instead of normal heating mean you make a bit of money instead of having to pay for heating.