This is exactly the question I've been asking myself, and tracking with calculations..

Someone on this forum mentioned that mining hardware is worth no more than $.30 per 1MH/s so in theory $300 per 1GH/s.  That's before electricity costs are factored in.  

However if you project difficulty into the future at about 10-15% increase every 2016 blocks, and then figure out how much you can mine, it is eye opening how few bitcoins you can actually mine in the lifetime of this hypothetical 1GH/s ASIC device.  Today you can mine something like 0.14 BTC per week, per 1GH/s, but 6 months later its 0.002 or 0.001 per week and it no longer matters if you keep mining or not, you're basically going to turn off the device and throw it away.  

And that's before you look at the market price of BTC .....

Edit: based on the difficulty calculations, a 1GH/s device will mine 1.6 BTC in 52 weeks starting from today, if difficulty increase trend continues at the average rate it was recently.  That's $160 if BTC price is $100 on average, $80 if BTC price is $50 on average.  (!)