Don't get a quote on having the windows replaced, except perhaps by a local contractor in the off-season when they're looking for work. There are tons of window companies out there that offer their own financing, etc. I've yet to run into one that wasn't a total rip-off. Twice in our old house, we had very high pressure sales pitches; I nearly had to throw one of the guys out of the house - he all but refused to leave.
Anyhow, you might be surprised at the low cost of replacement windows from your local lumberyard or even box store (Lowes, Home Depot). You can, of course, get a lot of features that add a lot to the price. But, to simply go from single pane to double pane, and get a tremendous savings from decreased heat loss, it's probably far cheaper and easier to do-it-yourself than you might think. Check out some videos on youtube for replacement windows. There are tons if different types of installations; most are very similar for replacing single pane windows, but there are differences in installation if, for example, you have all brick walls where the windows are. Most of the expense of manufacturing windows is equipment/labor, not materials. If I recall, there are large ranges of "united inches" - the sum of the length and width, that all fall under the same price. If you can use a tape measure, hammer, drill, and screwdriver, you should be able to handle the job yourself; figure around $150 per window. There are a lot of possible upgrades to get, but more expensive doesn't necessarily mean it's going to save you money on heating or that it's necessarily a better window at keeping the heat in. And many upgrades don't make sense from an economic standpoint of saving money in the long run. E.g., you're unlikely to recoup the extra cost to upgrade to triple pane windows through heating savings for a couple of decades. Though, if you live in a noisy area, it could be worth it for the sound reduction.
Anyway, here's a Consumer Reports article:
http://www.consumerreports.org/cro/magazine/2014/10/how-to-choose-replacement-windows/index.htm
Note that it says it's a long time to recoup your investment - if you pay to have them installed. Simply using their numbers for your situation - single pane - it seems that it would pay for itself in a reasonable amount of time if you install them yourself. Though, if you keep the heat dialed back in the winter to keep your fuel bills lower, it's possible if not likely that with better windows, you'll raise the temperature, making it more comfortable. Thus, you won't necessarily get the financial savings, but you'll increase the comfort of your home.
Edit: oh geez, I didn't look at your picture. I think trapezoidal windows cost more for replacements. At a very quick glance, prices seem higher than I expected for trapezoidal. Though, I only looked at one company. Beautiful wall, btw.