You could make the excuse that you have to pick up your dry cleaning, but that won’t wash.
I could say my zipper got stuck, but I don’t think that would fly.
mmm
I was thinking that you could say you’d be in Dayton that weekend, making a pilgrimage to Orville and Wilbur Wright’s original bicycle shop.
But I decided against it … for the very same reason.
You have to rake your yard, and leave it at that.
In the spirit of smart ass answers:
“I will gladly give your son’s music program a donation of $70, and then you can donate $70 to my son’s (baseball team, arts program, scouts, etc)”
Seriously though, it sounds like this is their fundraising concert. If you want to support, but don’t want to go, why not buy a few tickets and gift them to an elementary school band program closer to the concert venue? Or maybe to a family homeless shelter? Someplace where kids who need inspiration might get the chance to see a concert they otherwise wouldn’t.
It was prob blanket invite to local friends, feel free to show interest 
but decline to attend. Really they’d be delighted and surprised if you showed but it’s not going to put a strain in n your relationship.
No reason to start lying now.
Yeah, this. I feel the same way as @MandaJo about people who never give explanations in general, but at the same time I would not feel that “come to my kid’s fundraiser/concert” specifically needed an explanation to decline.
This is my preference. You owe nobody an excuse for most social things. Be polite and be honest.
Giving an excuse opens up the “but maybe you could…” discussion.
For example, “I won’t be able to drive there; my wife will be needing the car that week and mine has issues” would simply result in “Hey, I can stop by and pick you up!”
The fact of the matter is you don’t want to go. You don’t need to justify that to anyone. Just say “Thanks for including me, but I won’t be able to go.” and if they have a rejoinder, then simply say “Sorry, I won’t be going” and repeat that as necessary.
Or, if the $70 hurt to much, then just 35 “We can’t both go” and then “I couldn’t make it”.