If you can deal with the extra weight and bulk for your short hump in then you might be able to save some money by going with a synthetic bag. There is much more to a bag's performance in the field than simply the temperature rating. Zipper quality, draft tubes, type of stitching, etc. A -20 bag that has cold spots due to econo stitching be used only once. A -20 bag that doesn't ventilate or breathe well when used in temperatures above its rating will be used once. The life span of a quality down bag is far longer than its synthetic counterpart.
Temperature ratings for sleeping bags are second only to political speeches in terms of honestly or reality. WM has the reputation of being on the mark with their ratings. Few other manufacturers can say the same. MontBell is another popular line whose bags have a unique design allowing the bag to stretch somewhat thus fitting a variety of users better. Very popular and well regarded but I didn't care for mine all that much.
I'm a cold sleeper, 6'-2" and a skinny 170#. I move around a lot at night and sleep on my side. I am a sleeping bag manufacturer's nightmare
I spent years in roomy bags to facilitate my habits and was never warm because I had too much bag around me. I disliked mummy bags due to their confinement but finally conditioned myself to using one. World of difference. I'm warm and wear the bag which moves with me at night. I have a zipper that goes up and down as needed without snagging or binding. No cold spots and I can move the down fill around in my bag to customise insulation for conditions. Combined with my excellent mattress and a light base layer of clothing, my 20 degree bag will take me into the mid-teens comfortably.
When I'm doing the backcountry thing I need a good night's sleep in order to function properly and enjoy my adventure. Paying for my sleeping system was painful but I surely do it all over again given the results I now enjoy. And I won't ever have to do it again.
I strongly caution you against trying to economise this aspect of your kit but I do understand budget constraints. Best of luck to you in your venture.