I have attended fire training seminars in the past. The fire officials told our class that the cheapo $8 alarms from CT are just as good as the $30 ones. There are standards that all alarms must meet.
October is fire safety month, alarms generally go on sale then. Guess you will have to suck it up and spend a few more dollars now.... That shouldn't wait till October.

In regards to the lifespan. Generally 10 yrs, but I usually test it every 6 months. I light an incense stick and let a little smoke drift into the sensor. It the alarm doesn't go off within a second or so I replace it.

Note: Carbon Monoxide detectors are only good for 5 yrs.