unless you go business class i'm pretty sure all airlines allow 1 free checked bag this days
we use Westjet for our traveling and for second bag they charge $20, and its the same fee for scuba gear
we just used west jet into coz and back from cancun. it was reasonable cost like you said. 1 free plus 20 fro second (that is 20 each way of course) but west jet also allows 2 full size carry ons. not just 1 carry on plus a "personal" item. so we bought the biggest carry ons allowable (and fill them as much as possible) and also brought a pack sack each. we use the packs to carry the regs and other stuff. then check one bag for the rest of the scuba gear and one for clothes etc.
it would be impossible for us to pack for 2 weeks and get it all in two checked bags along with dive gear. i am glad they let us bring those carry ons.