It's not the cost, its the value you get!
Round-trip car service is usually around $100 or so (give or take a few $$ depending on what company you use). So for 2 people that's about $50 per person, or roughly twice what you spend on Mears.
What you get for that extra money, though, to me more than makes up for the extra expense! When you get off the plane, all you do is go claim your bags and meet the driver. He'll help you with your bags to your waiting car and wisk you straight to WDW. You don't need to wait for other passengers to fill up a bus, you don't make any stops at any other terminals or hotels on the way.
If you want, you can also request a FREE stop at a grocery store on the way to your hotel to stock up on snacks and drinks. This will also save you money, as you can buy a case of water for what one or two single bottles will run you at WDW. Same with soda, beer, chips, cookies, etc.
So between the convienance and money you save on snacks (if you choose to make a stop), I say the extra cost of a car service is a better value than cheap-o Mears!