To chip in. It means to contribute a small amount of money to a pool that will be used to buy something. The translation to this expression would be something like "To make a little cow", and the reason why a cow is used in this expression is probably associated with the cow animal from the Jogo do Bicho.
- Vamos precisar de 100 kilos de isopor pra fazer a maquete.
- Hmmm. Acho que vamos precisar fazer uma vaquinha pra comprar tudo isso de isopor.