What size quilt top do you want to make? I just finished a Falling Charms quilt. I think I had 3.5 yards of background fabric that I cut into strips. The top finished up at 50"x62". At the beginning I figured out how many strips I could get out of the background fabric that I had and figured out the quilt size from that.
The next time I make this pattern I'm going to cut the background strips at 2", as I think it will look better. I don't like quite so much "blank" space in my quilts unless I am going for a very modern look (which in my case would probably require white for the negative space).
I have always found it less expensive to buy yardage and cut it into strips myself than to buy jelly rolls. I used Kona Snow for the quilt I made, which is $6/yard online from Hancock's of Paducah and they have a $5 flat shipping rate now. That would be $35 for five yards of background fabric delivered.
For comparison, here are jelly rolls from the Missouri Quilt Co:
http://www.missouriquiltco.com/shop/...now-jelly-roll
Two of these would total $65 plus shipping for 5.5 yards.
Here is the same fabric in yardage at Hancock's of Paducah:
http://www.hancocks-paducah.com/Item--i-M-9900-11
which is $6 yard. 5.5 yards would be $38 (shipping included) if you cut your own strips.