Saw this on SlickDeals and figured I would share.
I am not sure if these work with SmartThings as I do not have them but they are Z-Wave so maybe someone here already owns one and can verify.
Z-Wave Smoke Detector $15.75 After Promo Code AUG10 at Monoprice
Here is the SlickDeals post:
http://slickdeals.net/f/8035115-z-wave-smoke-detector-15-75-after-promo-code-aug10-at-monoprice
and here is the direct link:
http://www.monoprice.com/Product?c_id=122&cp_id=12212&cs_id=1221201&p_id=10797&seq=1&format=2&AID=11064683&PID=4485850&ref=cj&utm_source=cj&utm_medium=11064683&utm_term=Slickdeals+LLC-1122587
I am holding out to get a few combo smoke/CO detectors when I have enough saved 
Hope this helps someone!
Scott
Edit: Oops, just went to search this and found that @cuboy29 beat me to the post by 20min