Hi Miggi03,
This should be easy to do. You can convert strings to numbers using the number() function. In addition, you can sum a column using the sum() function.
So, to extract the total of a column, just use sum(field) where field is the column in the repeating group or section.
You can use default values for the column to auto calculate the fields in advance before summing.
I'm a little confused about exactly what you are trying to do, so if this doesn't help, please email me.
Good luck!