OKie,.. first question, why are you converting everything into a currency?? the month and week variables should be ints.
Your overflow is occuring because your converting (or your calculation) is coming up with an invalid casting... eg. 123456789012345678 is too much for an int value.
You should be doing your calculations on something else like a double and then converting the result of the calculation to a currency.
Acutally your overflow is coming because you haven't defined time (edit: the same goes for amt). You are requesting it but it doesn't exist in your form. So you week and month variables are null. When you convert null to a currency it will overflow (a bit of a guess here).
Having said that, all the other stuff written above still applies.