Starrett squares come in several variations. Cast or forged heads. Polished steel or satin chrome blades. Reversible or non reversible protractors. All these options affect price.
The set you linked to isn't what they say it is. Forged heads are smooth finish, not wrinkle and 434 series not 435. All protractor heads are cast, but come in smooth or wrinkle finish to match the square and center heads.
I always recommend people go to the Starrett website to see the options before purchasing any Starrett tool, as many come in different flavors. If you don't understand the options, call customer service. Without an exact part number you can't get what you want.
Lots of internet sellers advertise the lowest priced option. I bet that is what the set you linked to is.
The Snap-on set is forged, chrome blade. If the protractor is reversible, with the case that lists for about $325, so at half price it's a great deal.
Here is the Starrett square page, sort it down to 12" sets, 4R graduation to give you an idea of how this all works.
http://www.starrett.com/category/precision-measuring-tools/combination-squares/111001#currentPage=1&displayMode=grid&itemsPerPage=24&sortBy=wp/asc
