Hi @jdelacrettaz,
You could take use of the Sum function here.
For example, the formula of the BrowseGallery Items property is the following:
SortByColumns(Filter('Testing list', StartsWith(SText, TextSearchBox1.Text)), "SText", If(SortDescending1, Descending, Ascending))
Then under the Count total Label Text property, the sum formula for the Number field should be:
Sum(Filter('Testing list', StartsWith(SText, TextSearchBox1.Text)),
Number
)
Adding the code suggested by @Meneghino:
Sum(BrowseGallery1.AllItems,
Precio)
Which should be better.

Adding the function reference:
Average, mAX, mIN, stdevP, Sum, and Varp functions in PowerApps
If you need any further assistance on this, please post back.
Regards,
Michael