Hi @MiguelRichneyD,

It seems the key here is where the set up of the computation (.TotalBookedDays*.RoomInfo.Rate) exist.

Is it in a data transform or a activity.

To achieve your requirement using data transform. I compose the computation as shown below

Create a Data Transform > choose set method and select target property > Then click on grear icon at end of row

The following screen will appear, click on the browse link

Select math > MultiplicationExchange method as shown below, click on plus icon to get the method on work area

Compose the method with properties as shown below and click submit

It will be shown in data transform as shown below

Save the data transform.

Now, call the data transform on the change of the "Rooms" dropdown, Configure the action set as should be like "Refresh-this-section with Data Transform" or "Refresh-other-section with Data transform". Refresh of the section where total cost present in the key. Without refresh we dont see the computed value on the screen. 

Hope this helps.

Please feel free to reply with questions if any.

Thank you,

Regards, Cherb.