$ awk 'BEGIN {FS=OFS="\";\""} NR>1 {$3 = sprintf("%.2f", $3+125)}1' p.txt 
"Productno.";"Description";"Price";"Stock";"Brand"
"/5PL0006";"Drum Unit";"504,29";"10";"Kyocera"
"00096103";"Main pcb HUK, OP6w";"1007,00";"0";"OKI"
"000J";"Drum, 7033/7040 200.000";"4431,00";"0";"Minolta"

Note that this requires a value of environment variable LC_NUMERIC that expects , as the decimal separator (I had mine set to LC_NUMERIC="de_DE", e.g.).

For more DRYness you can pass in the amount you want to add with -v:

$ awk -v n=125 'BEGIN {FS=OFS="\";\""} NR>1 {$3 = sprintf("%.2f", $3+n)}1' p.txt

If you don't care so much about the formatting (that is, if "4431" instead of "4431,00" is acceptable), you can skip the sprintf:

$ awk -v n=125 'BEGIN {FS=OFS="\";\""} NR>1 {$3+=n}1' p.txt  

EDIT: Set FS and OFS in BEGIN block, instead of independently via -v, as suggested in the comments (to better ensure that they receive the same value, since it's important that they be the same).

Perl to the rescue! Save as add-price, run as perl add-price input.csv 125.

#!/usr/bin/perl
use warnings;
use strict;

use Text::CSV;

my ($file, $add) = @ARGV;

my $csv = 'Text::CSV'->new({ binary       => 1,
                             sep_char     => ';',
                             eol          => "\n",
                             always_quote => 1,
                           }) or die 'Text::CSV'->error_diag;

open my $IN, '<', $file or die $!;
open my $OUT, '>', "$file.new" or die $!;

while (my $row = $csv->getline($IN)) {
    if (1 != $csv->record_number) {
        my $value = $row->[2];
        $value =~ s/,/./;
        $value = sprintf "%.2f", $value + $add;
        $value =~ s/\./,/;
        $row->[2] = $value;
    }
    $csv->print($OUT, $row);
}
close $OUT or die $!;

You can also use php and this fantastic library :https://github.com/parsecsv/parsecsv-for-php :

Download first the library, add it to a new folder and add a copy of your CSV file to the folder (make sure to use a copy, the save method of this library can delete the data of your csv file if you do not use it properly) :

With this library you can parse and modify directly the values !

<?php
// !!! Make a copy of your csv file before executing this 
// Require the Parse CSV library , that you can find there : https://github.com/parsecsv/parsecsv-for-php
require_once 'parsecsv.lib.php';
// Instanciate it
$csv = new parseCSV();
// Load your file
$csv->auto('data.csv');

// Get the number of data rows
$nb_data_rows=count($csv->data)-1;
// Iterate through each data row.
for ($i = 0; $i <= $nb_data_rows; $i++) {
    // Define the new Price
    $new_price=$csv->data[$i]["Price"]+125;
    // Format the price in order to keep two decimals
    $new_price=number_format($new_price, 2, '.', '');
    // Modify the ith value of your csv data
    $csv->data[$i]=array(
        "Productno."=> $csv->data[$i]["Productno."],
        "Description."=> $csv->data[$i]["Description"],
        "price"=>$new_price,
        "Stock"=> $csv->data[$i]["Stock"],
        "Brand"=> $csv->data[$i]["Brand"] );
    // save it !
    $csv->save();


}