It depends on if you do it or pay a garage. You figure average hourly labor rate is $70 - $80 plus cost of parts at list price. List price for OE Plugs could cost $12 - $15 each.
If you do it it you can save quite a bit. It a 4 cylinder the plugs are right on top and easy to access. You go to a auto parts store and purchase the parts you could save as much as 30% - 50%. And all you need are Spark Plugs and Wires plus the couple tools to remove the Plugs.
Also if you still have the engine code post it here.