Air bag in front passenger seat deployed in accident. Can it be replaced?

The airbag can be replaced, but it is costly because it is a complicated circuit. It includes;
Airbags
Seat Belts
Fuel Pump Cut-Off Switch
Interia Sensor

Then the circuit must be tested and signed off on by certified techs that even dealerships bring in as there is an insurance liability issue involved.

It is nearly impossible to estimate the costs with all the variables and I don't know the extent of the damage.
You are generally looking at something you want insurance to cover as it can run from $1000 to a few thousand, (unlikely though), so look for $1500 to be the average. This also depends on where you live. The best bet is to call a body shop. You can use the search engine on this site to find reputable dealers near you.