On a $/hash basis, the ASIC are cost competitive. R9 280x are around $450-$500 now and hash around .730Mhs, so for 3.5Mhs, you are looking at $$2,750-$3,000 for the cards only.
I hope these aren't the kind of calculations you do when working out potential returns. Not only do your numbers actually give $2150-$2400 for the cards needed, but I doubt many intelligent people pay $500 for r9 280x. Dock the $240-480 you think the cards will be worth in the end and the savings are still enough to power your cards for quite a long time (your numbers give $100/month as the difference in energy costs, giving 'free GPU running time' of 3.5-8 months worth using your numbers. I pay <$350 for 280x, I imagine most do, so there is another few hundred bucks for CPU/RAM/Mobo and still more spare for yet more electricity).
If you think these Scrypt coins will last more than a few months, then maybe this is for you. But I don't think they'll keep their value much longer, and soon people will move onto n factor or something else because Scrypts main draw is lack of ASICs. The bottom line is you need to have your 3.5MH/s ASIC hashing away for half a year before you can save as much on electricity as the extra you spent on your miners. Will Scrypt last that long? Will these ASIC chips last that long?
