The food is going to cost Calum a total of 10 boxes of food * $25 per box of food = $<<10*25=250>>250.
This leaves $330 budget - $250 spent on food = $<<330-250=80>>80 to spend on disco balls.
Calum can therefore afford to spend $80 remaining budget / 4 disco balls = $<<80/4=20>>20 on each disco ball.
#### 20