I am all about using alternative airports when appropriate, but the OP should consider the added cost of getting from NYC --> PHL and from TPA --> MCO. At $236, s/he is saving about $100, but it would be pretty hard to not spend that same $100 commuting round trip between NY and Philly and between Tampa and Orlando.
If the OP's dates are flexible, it would appear that it is the Sunday departure that is driving the price of the ticket. You could shave about $75 off the cost by flying ex-NYC on Monday, 8/23, and shave over $100 off the total by flying 8/23-31 instead of 8/22-30.
This is the last week of possible vacation time for those that have families before school starts up north, so flights NY-FLA are generally not cheap at this time.