If Cabinet needs to know what bills or programs are going to cost, it is perfectly natural that, before making its decision, the House of Commons would also have access to that information, even though it may be approximate. That is part of the debate.
You are right to say that it is not the whole debate, but I would like to cite the example of employment insurance.
Every time we propose enhancements to employment insurance, the Conservative government says that it will be too expensive, and that option is completely ruled out. However, if we need information about costs in the context of parliamentary debate, it would also be perfectly normal for us, like Cabinet, to be given that information, even if the cost is approximate.