You can create switch case statement to set a variable to the correct value, then bind the column to that variable in the DA Update Record server behavior.
For example, if the number of rooms needed is coming from a form element named "numberOfRooms":
<?php
if(isset($_POST['numberOfRooms'])) {
$rate = "";
switch ($_POST['numberOfRooms']) {
case "3":
$rate = "100";
break;
case "4":
$rate = "150";
break;
}
$_SESSION['rate'] = $rate;
}
?>
make sure this code is above the Update record code. No go to the bindings panel, click the plus button and Select Session Variable set the name of the session variable to "rate"
In the update Record behavior, you can bind the rate column to the rate session variable.