As far as how many sets of lights you can use. You can measure the load. Each set of 100 miniature lights draws approximately .33 amps. Therefore, 3 sets of lights draw 1 amp. Use simple math, 60 sets of lights would draw 20 amps and probably overload your breaker.
You can also calculate cost using the formula: watts = volts x amps Assume that you installed 30 strings of lights which would typically draw a total of 10 amps. 1200 watts = 120 volts x 10 amps. Now, look at how much you pay per kilowatt hour for electric. .06 per KWH means that running a 1000 watt circuit for 1 hour costs you 6 cents. Calculate how many hours you run the lights per night x number of days in the season. You get the idea.