Kudos for trying to do the right thing by spaying a stray cat. However, considering all that a spay involves, $120 is a perfectly "reasonable" price for major abdominal surgery.
http://www.thepetcenter.com/sur/costspay.html
http://www.shoalspaws.com/
http://www.alvma.com/displaycommon.cfm?an=1&subarticlenbr=194