Hi @Eyal_Orbach,
Thank you for the picture.
So a couple things, first I would not use the 2nd row to hold the sum of your data. If you want to keep it in the same sheet I suggest you putting it in a new column and name that column sum (putting the actual sum in the second row).
Next to have the sum range expand when new data is added you should be using an array formula which will expand to the size of your spreadsheet. So instead of sum(B3:B30) you would put arrayformula(sum(B2:B)). B2 because we are getting rid of the sum in the 2nd row and B2:B as the array formula will sum the whole column. I also suggest deleting all unused rows currently 5 onward. (when a form is submitted it will automatically add a new row)
Now you should have 2 columns Item and cost which have no extra data and a new column called sum which contains a single cell showing the total cost.
To display the sum and the list you no longer need any filtering based on column 1 either.
Let me know if anything above needs more detail!