If the price of a Bitcoin remains around $14 then is it worth buying a mining rig?

What I am thinking...

I was going to build two mining machines costing in total £2'500 (or around $4'100).
The builds' consist of six 6970s so I would be aiming to achieve a combined hash rate of around 2400 mhash/s .

My electric is charged at 12 pence (around 20 cents) per kilowatt hour.
My six GPUs will use around 200 Watts each. So that's 1.2kWh for them, plus, say another 0.2kWh for the two machines.

So that gives me a total of 1.4kWh. So now it's just a case of multiplying the amount of kilowatt hours by the amount I pay for my electric.

1.4 x 0.12 = 0.168

That means I'll be paying around 17 pence an hour, that's £4.08 a day, £28.56 a week and around £114.24 a month.

So considering my hash rate I should earn around £420 (or $700) a month on average.

That leaves me with an estimated £300 a month profit. It would take 8 months just to pay back my start-up costs.

Is it worth going ahead and buying the components or would you say that with the price of electric where I live it isn't worth it.

I suppose what I am really asking is, are these sort of margins acceptable and if you had access to the money would you go ahead with the above plan?

Please help I can't decide.