Your approach - internet pricing of parts - is not totally realistic. When you contact the garage for an estimate, they will not shop around for a bargain cat for you. Whether you go to Dodge, Midas, or Joe's Garage, the service manager will simply look up the price of the part they always get from their standard supplier. Then they consult the flat rate manual for labor charges. Even an honest shop will give you a pricey estimate and still be totally fair.
If you are determined to save as much money as possible, order a cheap cat from wherever. Then try to find a mechanic willing to install it for you. Some will, some won't. The mechanics face warranty issues. I used this method for my Dodge sedan and the total expenditure was something like $170, parts & labor.
But I repeat, higher repair estimates do not necessarily imply ripoff.