It would depend on size. Looking at Amazon
it looks like the prices range from a bit under 20 to over 60 depending on size/quantity (and this is inline with what I've seen at places like Costco). Some fire extinguishers are also "refillable" while others are not.
In terms of expiry - I've seen places say you should replace a fire extinguisher after 10 -15 years if it goes unused.
I would recommend putting a fire extinguisher in your kitchen (in case of accidents) and then depending on the size of your home, maybe 1 or 2 more in places like a garage (if applicable) or backyard (if you grill) as needed.