As per the above, you only get that price per 10 if you buy 50 (or more).

You'd have to compare 50x DM2 vs 5x HD7970.  The theoretical parity period is the same, but the initial investment (for either option) a fair bit higher.
I.e.
1. your comparison (note that I don't have all the variables you used, so give or take..) - investment: $612 - parity in 357 days
2. actual comparison to get that price - investment: $3,060 - parity in 357 days
3. if you truly bought only 10 ($70.20/each, per website) to match a single HD7970 - investment: $702 - parity in 464 days
http://imgur.com/a/YP5qk
( bitcointalk.org's image proxy is having a fit, so album link it is )

Of course one might be able to get a better deal on 5x HD7970, the resale value of graphics cards after a year is probably better, the static values are unlikely to remain static, etc.

Still, it's a pretty nice pricepoint