First Alert ONELINK Wireless Smoke Alarm (2-Pack) for $39.99.  Shipping is $5.00 for a total of $44.99 shipped.  Here is a screenshot for your reference.  It features15MHz, 65,000 security codes and 3-channel frequency, loud 85-DB siren, FCC compliant for wireless device standards, and more.  Most places have it for about $89.