from what I have read,it seems all that is needed to convert a 496 mag into a 496 ho is a cam and reprogram the computer.all other parts are the same ,is this true? if this is true ,why the huge price different between the two?

If you are talking about an engine-drive package then you will also get the Bravo One X drive with the 496HO compared to the std Bravo One with the 496.
Apart from that I guess Mercury is just trying to make more money on the 496HO than the 496.

I believe you are correct; cam and ECU program are the only differences. The early HOs came with a forged crank though.
I wonder if you could just throw a set of 1.8 ratio roller rockers on the mag, have it reprogrammed, and make the same power???