"I'm using editmask control with MaskDataType as decimalmask!"
I'm curious why you are using an EditMask control instead of the DW control to input the value. The purpose of the SP Update DW should be to input values directly to the DW and issue an update which would execute the insert SP with the corresponding argument, shouldn't it?
So how it is that you pass the value from the EditMask control to the DW and execute the SP?