Hi George,
Thanks for finding us.
If you are using InfoPath 2003 or 2007, just right click the C1 box and choose properties. Select rules. Add a rule with a condition where C1='$' and add an action to set the value of field T1 as 300. Add another rule with a condition for when C1 is a pound symbol. Easy to do.
If you are using InfoPath 2010, find the Manage Rules button in the ribbon and do it through the taskpane.
Good luck!