Have you ever compared prices to get the highest quality with the lowest price. I did that with a toilet brush the other day and found that I really should have gone for the $7 one the first time because the $1.50 was a waste of precious money. Lesson learned.
Sleeping bags can be the same way. Walmart can carry some good bags...unless you need to see a night under the stars under 40 degrees because of an emergency evacuation. Get it right the first time and don't waste your precious money.
My suggestion is to get a four season sleeping bag the first time around because you can add or take away bags to fit the season. I found used military bags for a good deal on Amazon at this link:
-listing/B00474ANDO/ref=sr_1_ 2_olp?s=sporting-goods&ie= UTF8&qid=1375851856&sr=1-2&key words=four+season+sleeping+bag &condition=used
To be honest...the price isn't that much different either. I have spent hours looking for bags and even the most expensive cannot guarantee comfort even around 20 degrees (that is above zero). Give it a good thought, because our pioneer fore bearers would have really enjoyed having these bags for their journey.