At this point why don't you just post the model number of the tractor and the model number of the engine and we'll do the rest.
You could go down to your local lawn mower shop and the parts counter people, using the numbers off the engine, can give you a price on the starter. Unless Briggs has changed their way of identifying their engines (which they may have) there are three sets of numbers to identify that engine. Usually on the shroud. Take all three with you. Sears is just going to mark it up in price.
The least expensive way, may be to find a starter repair shop in your area and take it in to them.