The materials for the more expensive kit would be more breathable, but at this time of year the UV rays are also a factor. The more expensive kit might arguably last longer and have better pockets and features.
Here's a handy guide on kit
http://www.cyclingweekly.co.uk/news/latest-news/buyers-guide-summer-cycling-clothing-175176
Thanks for the link but that's probably the shallowest piece of writing disguised as a "guide" I've read in a long time. It says nothing.
I would have liked to see them compare polyester to polypropylene, for instance. Although it mentions UV protection , the advice that a net is less protective than a fabric, is hardly not common sense. Again, it would have been nice have some advice on how some fabrics and/or colours provide more protection than others and how things like weave density is measured and gauged by the consumer.
Anyway, nowadays all seem to be made from polyester, a very stinky material. Fabrics like Thermavent which were half cotton half polyester (cotton below, polyester above) seems to have disappeared. Pity that.
The only difference I see nowadays between expensive and cheaper jerseys is in manufacturing quality, not fabric properties. Nicer seams, hidden zips, secret pockets, great prints etc, are worth paying for, but choice of fabric is gone, unless you can afford Rapha.