That's a pretty big assumption, that bitcoin will return to $15-$20. The trend over the past 3 months has been nothing but down. Do you feel confident that enough new adopters will be found for bitcoin to drive the price up? What do you see as the catalyst for this happening, aside from difficulty rate which is irrelevant to bitcoin's value?
I feel like bitcoins will either be a huge success and go main stream or it will roll over and die. If it is a huge success and goes main stream it most likely will not stay at around $10 a btc. I'm here for the long haul and I'm willing to be patient and wait how ever long that may be. I'm not going to be the dude who spends 10k bitcoins on a pizza. I realize there's probably no chance for a jump that high but it's entirely possible people are selling for cheap now and in a month or two it will be back up.
In regards to bitcoins failing I don't mind because I'm a computer science major so this is my hobby. If I lose a grand or two it's not the end of the world because I thoroughly enjoy doing this.
There are a couple of differences here. Investing in the shares of a company is not like investing in bitcoin: the company does something and creates value. Bitcoin is just a commodity. Graphics cards are essentially worthless after 2 years. Increasing DirectX versions make previous card version obselete. A card may pay off itself in 4 months, but don't forget the case, PSU, CPU, ram, storage, and motherboard required to make that card work. Payoff time is now much longer.
The graphics cards may become valueless over time but I'm pretty sure a lot of people would be willing to take a 2 year old corsair psu thats gold rated. As long as you have some quality parts you will be able to get money back.
As for how long it takes to pay off a whole rig. My 5850's can give me enough bitcoins to be paid off in around 60 days. That's about half the cost of the server so in another 60 days I'll have enough bitcoins to finish off the costs. That's 4 months which isn't bad at all.