Because the child's personal computer resides in their room. This means less room (no pun intended) for control over active times, more costs involved because of computers not being shared, and also enforces the child to be excluded from social environment. It is a double edged sword. Both options have their pros and cons, and none is ideal.

Why would it be in their room. We have all our computers in one room, our family office. A shared, social environment.

You're right.

This seems like a hard problem, esp. since OP is so emphatic that $300 is preferrable to $500. Let me see if I can figgure this out for us both.

1 child x $500 = $500

1-or-more x $500 = $500-or-more

Oh! That must be it! This solution would be more expensive to implement, and therefore, since cost is a factor, this solution would be less preferable!

Glad I took the time to work this one out, I certainly was very confused as well.

Money isn't always the primary factor.