Here are  low, middle and high end monitors from Amazon. They usually have pretty good prices and lots of customer reviews:

Low:  Omron HR-100C Heart Rate Monitor!http://www.assoc-amazon.com/e/ir?t=cts1230-20&l=as2&o=1&a=B000A5CEUO|height=1|style=border:none !important; margin:0px !important;|alt=|width=1|src=http://www.assoc-amazon.com/e/ir?t=cts1230-20&l=as2&o=1&a=B000A5CEUO|border=0!

Middle: Garmin Forerunner 305 GPS Receiver With Heart Rate Monitor!http://www.assoc-amazon.com/e/ir?t=cts1230-20&l=as2&o=1&a=B000CSWCQA|height=1|style=border:none !important; margin:0px !important;|alt=|width=1|src=http://www.assoc-amazon.com/e/ir?t=cts1230-20&l=as2&o=1&a=B000CSWCQA|border=0!

High: Polar RS200sd Heart Rate Monitor Watch (Black)!http://www.assoc-amazon.com/e/ir?t=cts1230-20&l=as2&o=1&a=B000H2AZM4|height=1|style=border:none !important; margin:0px !important;|alt=|width=1|src=http://www.assoc-amazon.com/e/ir?t=cts1230-20&l=as2&o=1&a=B000H2AZM4|border=0!

Crazy High: Polar RS800CX Multi Sport Heart Rate Monitor Watch with G3 GPS Sensor W.I.N.D.!http://www.assoc-amazon.com/e/ir?t=cts1230-20&l=as2&o=1&a=B001F0PVGM|height=1|style=border:none !important; margin:0px !important;|alt=|width=1|src=http://www.assoc-amazon.com/e/ir?t=cts1230-20&l=as2&o=1&a=B001F0PVGM|border=0!

Good luck.

I know a lot of folks are turned off by the foot pod, but I have a Forerunner 50--relatively cheap and I like the size on my wrist better than the Forerunner 305.  Plus you can wear the wrist unit for the swim.  I still am not brave enough to wear the HR monitor on the swim, but I have seen some folks do it and get away with it.  I also have the optional bike sensor, so I am able (with a little manipulation) to record my times (and other data) for all three disciplines.