They have already had budget issues. They cut a planned battle from season 1 by only showing the aftermath when Tyrion woke up, they cut a battle scene from this episode and they had to ask for a bit more money to do the Battle of Blackwater in episodes 8 and 9 justice without the wildfire looking like cheap CGI and so they had enough extras.
such is TV
its been a while since I read book two....how does Blackwater get going? Stannis launches the assualt I think?