I don't think it would cost more to make it, b/c knit fabrics are usually 60" wide, so you would split it down the middle, then join it at the end to the first piece and thereby only need to buy half the yardage (3.25 yds.) wazoodle.com and several other places have cotton jersey for around $3.50 or less a yard, btw. I don't know how much Moby's go for new, but I'm making myself a homemade Hug a Bub, which is also a stretchy wrap sling, and they go for around $70 or more new -- I'm definitely saving money by making it!
you can find nice woven fabrics in your local fabric shop -- I recently made a woven sling from a yarn-dyed cotton stripe fabric that had no "wrong" side, looked Guatemalan, and I bought it at JoAnn fabrics on sale! Good luck finding something to suit you!