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In 1/06 I bought 4 new Goodyear P205/70R-15 tires from Discount Tires for $83 each.
  Recently I had my oil changed & an emmissions test at Fireston.  Firestone pointed out some cracking & unevenness on the sides of my tires altho treads were good; they said it might not be safe to go more than 30 minutes on highway driving.
  So I immediately went to nearby Discount Tire where I'd bought the tires.  They looked at my tires too, admitting there was slight cracking & unevenness on a couple of the tires.  They said this was normal due to the heat here in LV, but that it wouldn't be dangerous to drive 30+ miles steady on the highway.  However, they also said normal tire life in LV is about 4 years.  So altho my tire's tread is still good (should be after only 6,500 miles driven since 1/06, right?  FYI, I'm retired), they suggested I come back after this summer in about 4 months to have my tires looked at again.
  So my question(s) is, beings I don't drive many miles & heat is the main cause of tirewear (& batteries) here, when I buy another set of tires can I get by with buying cheaper tires?  (I imagine Discount Tire will be telling me later this year that I should replace my 4 tires)  

The cracks are an indication of the physical properties of the rubber in the tire.  As a tire ages, it becomes less able to flex, and the result is cracks.  So some amount cracking should be considered normal.  

Rubber is quite suspectible to heat.  My high school chemistry teacher told us that a 10 degree increase in temperature doubles the reaction rate.  So when it's 110 in Las Vegas, the rubber is deteriorating 8 times as fast as when the temperature is 80.

But this isn't really about tire wear.  It's about the structural properties of rubber.  Tires with poor rubber properties suffer from tread separations - the Ford / Firestone thing a couple of years back.

But buying cheaper tires might also mean buying tires more suspectible to heat.  I would trust Discount Tire to steer you in the right direction - and it might mean more expensive tires.