Okay, I'm trying to be fair and reasonable here.
It's 1979, so $50 is a ton of money (I think I got $5 a week at that time, and that included money for a drink at school). At 13, and with band auditions coming up, I doubt Henry has any way to pay for this.
Band is an activity, an elective. It has merits, but it's not a requirement. Between this, and Henry waiting until the last minute to get motivated to practice, I'm tempted to just tell him it's not happening, and he can drop band. After all, that would be a logical consequence.
I think what will really happen is I'm going to pay for this, but only after Henry and I have a long talk about being more responsible, and it will include him promising to practice more regularly. I'm also going to talk to him about the fact that he didn't mind me, and I'll ask what the kind of behavior customarily brings.
I think a spanking is probably needed here, all things considered, but probably not one on the harsh side - enough to make sure he knows he's been spanked, and probably enough to provoke some tears, will be enough to stress the discussion.