B..
Yeah, it was a different time and a different family and different circumstances when your mil grew up. She does not recognize any value to art. It doesn't pay the bills, it doesn't put food in your mouth, she just doesn't have a pocket to put it in. The jump rope, she understands that its play. She understands that it doesnt cost anything.
Just as you dont understand her responses, she doesn't understand your thinking. it's like you both are from a different planet.
I am sorry she is not supportive and might not ever really be. It would probably be best if you did not include her in the details of your life, especially telling her the cost of anything.
You might just say, we can afford it. Then you won't be disappointed and she won't be disapproving.
You can invite her to the dance recital but expect the same reaction and be pleased if its goes better.