[Carnage]

well, a problem i had with a game i wrote was that i was comparing the money to a value loaded when the script started part way thru the scripts execution you then update based on the value in the database which might have changed since you loaded it.

If you change your db query to update to a set value it will reduce the problem.
$test->db->query("update test_user set money=money-'{$Land_Cost}' where uid='{$test->user['uid']}'");

should be
$newmoney = $test->user['money'] - $Land_Cost;
$test->db->query("update test_user set money=$newmoney where uid='{$test->user['uid']}'");

a second way which may not be possible depending on your setup is to change your database engine to innodb and use row locking to prevent the data being changed while you are modifying it.