Most dealers include up to 5 quarts of oil in their oil change special. You will be charged for the 5 quarts whether they put all 5 in your car or not. They won't charge you less if you don't use all 5 quarts.
I would ask the service manager or check the fine print on the oil change special. I would bet charging out 5 quarts is just how their billing system calculates the price of the oil special.
The dealership loses money on every oil change they do, so they probably won't give you anything else for free.
Some dealerships are required to run national or regional oil change specials.
For example, at any GM dealership in the country, you can get an oil change with up to 5 quarts of conventional oil and a tire rotation for $39.95. Semi-synthetic, full synthetic and more than 5 quarts of oil are extra. I would bet his dealer is tied into a promotion like that.
The price on the oil filter is pretty high, too. I pay about $7 per filter and $2 per drain plug gasket at my Mitsubishi dealership. I use Eneos oil so I don't really worry about what Mitsubishi ants to charge for their oil.
The dealer I go to didn't have an oil change special, it was just a standard service. Honestly the most obvious reason they charge so much is because they can, next closest Mitsubishi dealer is around 20 miles away.
I've got about 5,000 miles on my Mirage and haven't gotten the oil changed yet. I'm shocked how much everyone is being charged! Oil changes were $35 at my local Nissan dealership every time I got my oil changed there in my 2012 Versa. I'm not about to drop $50 or more on an economy car oil change. That's nuts!
Was this your first oil change? Mitsubishi sent us a coupon in September for a free first oil change, and it is good until February. Like several people have suggested, I would encourage you to change it yourself. It's pretty easy and on this car should only take about 15 minutes due to the placement of the drain plug and filter. Even for a "top shelf brand" like Pennzoil, Mobil 1 or Castrol, you should only be paying about $35 - and that's for five quarts, two of which you can use later. I am doing my own every 7,500 so as to not void the warranty; once this car passes 100,000 miles, it will be every 15,000.