OEM is definetly the best way to go. The aftermarket ones leak in the exact same place, only sooner.
16410-YZZAJ has a list price of $289.32. They give you a discount off that price. $289 is super cheap in the first place, considering many mid-late 90's Toyota radiators were $300+ from Toyota.
The reason there is a difference in price between the dealer, and trademotion, is called dealer markup. List price is $289. They then mark that price up even further to "retail" price, which is above list price. Trademotion (aka Toyota of Dallas) discounts from list price. They buy from the toyota supplier at "cost" and still make a profit.