Help to afford fitness

Im a danish 14 years old and i ride bmx, my parents can only afford to pay my sport, and not quite that, and im not the biggest Guy, i want to go to the gym, but my mom says that we cant afford it. I really want to get stronger, i have always wanted to be big. It costs 269dkk a month. So it's about a year in fitness

I weigh 53 kg, is 160 cm talk, have 25% fat and 50% muscle says my weight (dont think is precise).

Thanks for your time:)

Hope you wil help