Many states give these items out for free and some even install them free of charge.
Many have qualifications such as age/income etc.
I posted a few. You can google Free Smoke detectors and there are tons of listings for this as many, many cities and states offer some type of assistance to get them.
At first I was going to try to put lots of them but as I searched, I realized there were far too many programs out there to try and list them all.
Here's a few to start some people off though.
Decatur, GA: http://www.decaturga.com/cgs_c.....ctors.aspx
Fresno, CA: http://www.fresno.gov/Governme.....rogram.htm
Long Beach, CA: http://www.longbeach.gov/fire/
Santa Cruz County, CA: http://www.centralfpd.com/AboutCentralFire
Terry Haute, Indiana: http://mywabashvalley.com/fulltext?nxd_id=150366
Walton, Fl.: http://www.co.walton.fl.us/index.aspx?NID=347
Austin, Tx: http://www.ci.austin.tx.us/fire/smoke.htm
Fort Worth, Tx: http://www.fortworthgov.org/fi.....px?id=2580