If there is no charge for extra then extra price can be set to 0

for example:

1 Extra=0
2 Extra=0
3 Extra=0

case II:

Lets say if , up to 2 extra are allowed . then after it's 50$ then you can do like :

1 Extra=0
2 Extra=0
3 Extra=50

If you want to set more than this then you can always use extra field parameter.