http://www.bitcoincharts.com/ they have a 4.52% jump so 6 or 7 is in the middle. I would love to see a solid drop in price to 175usd this would scare the shit out of the big miners the dc's even make knc and bitfury sweat.
Blocks 337001
Total BTC 13.675M
Difficulty 40640955017
Estimated 42478493256 in 1687 blks
Network total 321462.682 Thash/s
Blocks/hour 6.63 / 543 s
At $175/BTC I think we'll see older hardware dropped from the network quicker, but I'd say that there's still a commercial business model for a large-scale miner. At that price point the vast majority of the available mining rewards will be spent on electricity costs so there won't be huge amounts of new hashing coming online (I think we could see things difficulty increases drop as low as 2x per year; essentially just seeing the benefit of silicon process improvements).
If we assume that hardware operates at "x" J/GH and that they're getting power for "y" c/kWh and a nominal 300 PH/s total network and that the network really isn't expanding (so we have 144 blocks per day):
Generation of: 25 * 144 = 3600 BTC per day, so mean hash rate per BTC is: 300 PH / 3600 = 83.33 TH.
Power consumption of 83.33 TH/s: 83.33 * x kW, or 2000 * x kWh per day
Energy cost for our 83.33 TH/s is: 2000 * x * y
If we start to think about the marginal costs associated with hardware, let's guess that we need to cover costs in 365 days. I'll take Bitmain's S5 pricing for 52 units (60 TH/s): $17.25k. Let's assume that commercial users can get a huge discount of 50% so that's $8625, or $11979 for the 83.33 TH/s that we need to earn 1 BTC per day. That in turn means that they need to earn 11979 / 365 = $32.82 per day to cover the hardware costs.
In practice the difficulty will increase throughout the year so let's guess that we want to take $65.64 on day 1 and linearly scale down to $0 on day 365 (yes I know that in practice this would want to be a log scale but I'm doing the maths quickly :-)).
Now let's guess that the difficulty increases 2x this year. The S5 is quoted at 0.51 J/GH so in our earlier calculation x = 0.51. At the end of the year we're earning 0.5 BTC, or $87.5, per day so using our earlier equation we get: $87.5 = 2000 * 0.51 * y. y = 0.0858 $/kWh. Therefore if our miner can get electricity at 8.5c per kWh then they've been seeing a constant positive revenue over 12 months. On day one they receive $175, spend $87.5 on electricity and $65.64 to cover hardware costs; that's a net positive of about $22. On day 122 they're receiving $138.89, but only spending $43.76 on the hardware so they're earning $7.63. On day 244 they're receiving $110 but only spending $21.88 to cover hardware costs. It's not much of an income but it's still $0.62.
Now these numbers are rather approximate and don't consider any other costs that a miner might have, but they'd indicate that there's certainly scope for mining to be a commercially viable enterprise at even sub-$200 BTC prices, especially as hardware becomes more power efficient. If the hash rate expansion were to slow then the depreciation periods can increase even further, while lower cost electricity will definitely let things be more viable.