They may not be cheaper, but other products are Heat 'n' Bond Lite and Steam-a-Seam (regular and lite). I personally prefer SAS to Wonder Under because it leaves a softer hand to the fabric.
Your best bet may be to price a bolt online and look for the best deal. Before purchasing, Google "coupon code" name-of-store as you can often find a code for free shipping, etc.
I just went to
http://www.joannfabrics.com and found a bolt of Wonder Under on sale for $52.49 . Then I Googled and found the following coupon codes I could try on this order:
JUNFSA950 for free shipping (worth $9.95 to me) or
TTG184A for 10% off order.
There are many coupon code websites; I usually check a couple before placing an online order.
The bolt is 35 yards, so total price for me using the free shipping code would be just slightly more than $1.50 per yard.