Mysore has a good number of reputed cab operators. Go for a daily package for the local sightseeing. This is both convenient and economical , given the dispersed nature of attractions in Mysore.
Typically cabs services offer different rates for AC and non AC type cars.
A reputed car rental agency would charge you about Rs1500 for a day's city tour (8am to 8pm) by a non AC cab (Indica). For the AC type (Indica) it would be Rs1800. Four people can use this. The coverage include attractions in Mysore city and Srirangapatna.
If your destinations include places around Mysore (like Somnathpur, Belur, Sravanabelagola etc.), the charges are Rs2000 for non-ac and Rs2200 for ac Indica.
In case if you wants to hire a cab on a per km basis, there rates are around Rs7/Rs8 for non-ac/ac type India car. In addition you've to pay the driver allowance (about Rs200 per day, 8am-8-pm) and minimum distance payable is 250km per day.
The above are indicative figures for you to get an idea of the car rental scenario in Mysore. Check with a rental agency or your hotel for a better package.