While prices of flight tickets from Goa to Mumbai are extremely high on Sunday, Monday, when there is no holiday, from Tuesday to Saturday, the flight will usually be more empty as there are fewer tourists returning from a holiday to goa to rejoin their office. Most people prefer to visit goa on a weekend and then return to their home town on Sunday or Monday.
As the airlines try to increase the number of passengers on their flight, each airline has their own method to attract customers. For example Indigo will decrease the prices 36 hours before the flight timing, while Indian Airlines will drop the fares less than 24 hours before the flight.
So depending on the time when the travel plans are finalized, it will be possible to get the best pricing. More details of flight prices, variations at Asian Flights.