Tires are an interesting topic. Sometimes, you can really get what you pay for, others, you can really get ripped off. I've purchased many sets of tires from Discount Tire when we lived in an area they were available - after a bit of a history, they'll give you greater freedom to haggle prices. The last set I purchased were Cooper tires for the Volvo. The price at the local shop, mounted & balanced w/ stems was about $4.00 less than paying for the tires at Tire Rack, paying for shipping and having the shop mount them. Centerville is 4.98 square miles - for those that live in town, they're never far from home. I drive 104 miles each way, twice a week to attend class, plus the 188 miles each way to Kansas City twice a month. Lots of highway miles. Lots of rotating, too - every time I change the oil. I've been happiest with the Pirelli Scorpion tires I had on my truck.
The rule of thumb for trailer tires that I was taught is that they're good for about 5,000 miles or 5 years.