Submit Model doesn't send attributes that have functions

[axwack]

I have a viewModel model that has the following type characteristics:

Broker: "",  
BondProceeds: function(){                                       

           return this.get("UnderlyingPar") * (this.get("UnderlyingPrice")/100) + this.get("UpfrontFees");

                                        },

When I submit using Create on a datasource Broker will get sent but not BondProceeds. I suspect because there is a function. How do you work around this?