How to sum a field that numbers in varchar

[Kazantia]

I want to calculate a sum from a field that has varchar fields, the values are in this format ######,###.
The numbers before the comma can vary and the ones after the comma are allways 3. I get these values from a table from other database (so modifying that table is no option).

I know that I can't do a sum on a varchar, so i have to convert it
i tried different cast and convert statements but i allways get a error that says that the conversion failed

How can I do this sqlserver itself?

[Delerna]

you can't convert it because of the comma.
You can remove the comma with replace() before converting it

   convert(  decimal(18,3) , replace( thefieldname , ',' , '' )  )
 

[Kazantia]

Delerna,
Thanks for the quick response.
When i do this i get the number as a integer so the numbers behind the comma are now in front of it.

How can i solve this?
I tried to divide by 1000 but stll no numbers behind comma.

[Delerna]

I take it then that the comma is being used to represent a decimal point
in that case

replace( thefieldname , ',' , '.' )

should solve it