Math calculating in PHP take to much

Det understaaende script opdaterer total brugt credits i min database., Men det tager for meget i total.

credit cost = 1.00 credit
Naar jeg har 10 clicks 10 * 1 = 10, det goer den fint nok
Men naar den skal opdatere totalen tgaer den 45 credits i stedet for 10.

//Update Credit used

$result = mysql_query ("SELECT * FROM campains where campain='".$campain."'");
while ($row = mysql_fetch_array($result))
{
// get total clicks
$clicks= $row["clicks"];
//get click cost
$costcredits= $row["creditcosts"];
//get awaible balance
$credits1= $row["credits"];
//get creditsused
$used = $row["creditsused"];
  }
//calculate the cost
$cost=($clicks*$costcredits);
//Take the cost from balance
$credits = ($credits1-$used);

mysql_query("UPDATE campains SET creditsused  =$cost  WHERE campain ='".$campain."'");
mysql_query("UPDATE campains SET credits  =$credits  WHERE campain ='".$campain."'");

Fra start foelgende values var i Database tabellen:
credits     clicks     creditcosts     creditsused
1000.00          0        1.00            0.00


nuvaerende data i database tabellen efter 10 Kliks:
credits     clicks     creditcosts     creditsused
955.00          10.00        1            10.00

Som du ser tager den 45 i stedt for 10, hvorfor og hvordan kan jeg faa det korrekt.