I do not think you can cancel without losing your deposit, however it is worth going back to the company and explaining to them that you have found exactly the same package for less money and ask them can they offer some sort of discount? It is worth a try.
Go back to the company you booked with to let them know the price is cheaper elsewhere, they may offer to refund the difference or offer something else to cover the cost difference such as free insurance etc etc. But dont cancel, you will loose your deposit which will outway the difference in cost I guess.
All agents have may have different promotions, I am sure your agent gave you the best price possibl.