You are passing the prices in the post data as the checkbox values, but don't use them (they can be spoofed). Instead use the id to get the price.
EXAMPLE
<?php
$available_seasons = array (
226 => array (
'season_name' => '2022 - Season 1',
'season_price' => '25.99',
'season_start_date' => 'Jan. 1, 2022',
'season_end_date' => 'Mar. 31, 2022',
'prize' => 100,
),
227 => array (
'season_name' => '2022 - Season 2',
'season_price' => '28.99',
'season_start_date' => 'Apr. 1, 2022',
'season_end_date' => 'Jun. 30, 2022',
'prize' => 100,
),
238 => array (
'season_name' => '2022 - Season 3',
'season_price' => '40.99',
'season_start_date' => 'Jul. 1, 2022',
'season_end_date' => 'Sep. 30, 2022',
'prize' => 230,
),
239 => array (
'season_id' => 239,
'season_name' => '2022 - Season 4',
'season_price' => '30.65',
'season_start_date' => 'Oct. 1, 2022',
'season_end_date' => 'Dec. 31, 2022',
'prize' => 300,
)
);
if ($_SERVER['REQUEST_METHOD'] == 'POST') {
$total = 0;
echo "<h3>Chosen seasons</h3>";
foreach ($_POST['choice'] as $id => $chosen) {
$price = $available_seasons[$id]['season_price']; // get price from your data, not the user
$total += $price;
echo "{$available_seasons[$id]['season_name']} — \${$price}<br>";
}
echo "<h3>Total due  \${$total}</h3><hr><br><br>";
exit;
}
?>
<!DOCTYPE html>
<html lang="en">
<head>
<title>Example</title>
<meta charset="utf-8">
<script src="https://code.jquery.com/jquery-3.3.1.min.js"></script>
<script type='text/javascript'>
function checkTotal(box) {
var sum = parseFloat($("#total").val())
if ( $(box).is(":checked")) {
sum += $(box).val() * 1
}
else {
sum -= $(box).val() * 1
}
$("#total").val(sum.toFixed(2))
}
</script>
</head>
<body>
<form name='seasonForm' method="post" action=''>
<fieldset>
<legend>Tickets for Seasons</legend>
<?php
foreach ( $available_seasons as $k => $season ) {
echo "<input name='choice[$k]' class='uk-checkbox' type='checkbox' value='{$season[ 'season_price' ]}' onclick='checkTotal(this)'/>
{$season['season_name']}<br>";
}
echo "<button type='submit'>Continue</button><br>";
?>
</fieldset>
<input id='total' value='0' >
</body>
</html>