Maybe something like:

let t := this;
sum((select COSTO_W)[Week <= t.Week].Cost_Week)

You want to find all records with where the value in Week is less than or equal to the value in Week of the current record. This is assuming that you only have 1 record per week.

I had worked something out along Fred's lines, which can be adapted to your needs. If your table looks like this

Where the closing balance is opening balance plus adding less subtracting. In your Trigger after update put this code

let a := this.Order;
let b := last((select Table1)[Order < a] order by Order).ClosingBalance;
for c in (select Table1)[Order >= a] order by Order do
    c.(OpeningBalance := b);
    b := c.ClosingBalance
end

Regards John

Rafael said:
Hi Fred the Formula give me 0.00€

 So my first question is does the filter work properly? Like the last post we need to check to make sure you are getting the correct records.

let t := this;
concat((select COSTO_W)[Week <= t.Week])

If that gives you the proper records then you can add the field and see if it pulls the correct data.

Rafael said:
the formula give me this result, All Id's on COST_W

 Are you in the COSTO_W table? Can you upload a copy of the DB?

I put this in cost_by_week:

let t := this;
sum((select COST_W)[Week <= t.Week].Cost)

Hi both, I hope you don't mind me saying bit I'm not sure it is a good idea to have a select statement in a formula field

let t := this;
sum((select COST_W)[Week <= t.Week].Cost)

If you introduce a new record within the existing records, the later ones don't always cascade update

Plus there's the issue of multiple select statements in your code

Regards John

Yes, that's why it works for you. It also works if you only add new records to the end of the list but not if you are inserting records or updating them