Liza If you are really 44 then you are old enough to know the answer to your first question without asking. OF COURSE you are unreasonable if you spend ten times as much on shoes for one child than you do on the other. What does that say to BOTH of them about how much you value each of them? One ten times as much as the other?
You could replace your son's shoes ten times for every pair you buy for your daughter, so there is no need for him to go to school looking scruffy.
You might have to remind your daughter when she complains about favouritism that HER shoes cost ten times as much so they ought to last ten times as long, and that she can have new ones more often if she uses cheaper ones. I am sure she will understand that.
But you have carried on this discussion about nothing for so long that I don't think anyone is very interested any more, including you.