I have been a Hot Springs spa technician for several years.
The answer to efficient operation of your hot tub/spa is full insulation and a quality cover at least 4 inches thick.
NEVER shut the heat off. Re-heating the body of water repeatedly is senseless. Heat the water retain the heat with insulation and have the heater maintain the proper temperature. If you live in a colder climate turning off the heat will cause the tub to freeze and split the plumbing.
Upgrade your heating system from 110 volts to 220 volts if you can. 110 volt heat systems are less efficient tham the 220.
Ultimately older tubs built by Billy Bob in his garage should be replaced with a newer unit that is engineered and designed to be cost effective. I recomend the Hot Springs and Tiger River spas. The designs are tested in northern Canada and Alaska for efficiency in extremely cold weather. The warranty protection and service is second to none. Your tub should operate at $30 per month or less if its maintained and used appropriately.