B&H is usually quite competitive but not the absolute cheapest. I've bought a lot of stuff from them, sold them some used equipment, and have been happy with them.
They want [link|http://www.bhphotovideo.com/bnh/controller/home?O=productlist&A=details&Q=&sku=348425&is=REG|$500] while PriceGrabber lists them as $600 for the 750Z. It may be that other sites have lower prices as well.
Check [link|http://froogle.google.com/froogle?q=Optio+750Z&hl=en&lr=&tab=wf&pid=4701738945615954726|Froogle] too.
Be sure you're getting a US (or equivalent) warranty. Lots of places sell cameras without a US warranty and thus charge less for them.
Check out [link|http://www.dpreview.com/reviews/specs/Pentax/pentax_optio750z.asp|DP Review] for some information on it and 3 reviews from owners.
Luck! HTH.
Cheers,
Scott.