My humble .02 is that this is the one place where you just can't put the budget constraints on. I have bit the bullet on two nice down bags , and they have collectively lasted right at 25 years, and the latest one is only 5 years old.
Spend the money! You can feel the difference when you sleep, and you can appreciate the difference in weight. And in addition, the better down allows you to use a lighter rated bag in colder weather. I regularly use my MH Phantom (30 degree/ 800 loft)in below freezing conditins. With the extra clother I have anyway, its an easy way to add to my bags versatility. Its worth every single penny to spend an extra 50-100 bones! you will never never regret it.
Good Luck!
FYI, there are some great deals on high end bags right now.