You need to be aware that the amount that he is paying is considerably less than the UK CSA equivalent which would be in the region of €400 a month.
In fairness the CSA would agree a substantial discount for travel costs for contact
In addition the only way that the child maintenance figure can be altered is via the Courts in Belgium.
You may be more successful in challenging the travel costs.
You cannot be forced to have the child for every school holiday if you cannot afford to collect and return her.
Your husband needs to make it clear that unless his ex is willing to help with the travelling then the contact will have to be cut back to an affordable level.
Alternatively if can apply to the courts in Belgium for an Order that his ex contributes to the travel costs.
I hope that this is of assistance - please ask if you need further details