It depends based on where you are.  In Bonaire, the cost is around $350 here (if you buy an expensive mask) http://www.caribinn.com/diving.html .\n\nYour approach makes sense but will cost you more -- for example in Bonaire the open water dives would be $175.  And a class locally would be around $200 BUT you would also need to buy some equipment (mask, fins, snorkel, booties, etc.)