The cheapest way to lower the temperature is to get a rheostat (a "lamp dimmer" from Home Depot for about $12). That will set how much power is going to the heater, but will not adjust for changes in the air temperature, so you may have to adjust the rheostat periodically. The next step is to get a thermostat and the cheapest are around $30. To find out more, do a forum search for "rheostat" and "thermostat"
Aliza