You can get the kits cheaper online, but you won't have any gas. The CO2 bottles have to be shipped empty.
Single tap for $90 to covert a fridge.
http://www.beveragefactory.com/draftbeer/conversion-kits.shtml
To get CO2, go to any welding shop, or look up Roberts Oxygen. A typical tank exchange will be ~15 bucks. To get your fist tank, without a shell to exchange will run you ~65 or so for the core charge and gas.
From experience, try to buy a kit without a CO2 tank if possible as Roberts does not fill tanks, they only exchange them. There is no point in getting a special chromed or polished aluminum one, as you will just hand it over to them and it will disappear forever.