I think Mears is currently at 27/adult, and 19/child. So, I think, for a family of 4, a towncar works out cheaper at 80-85 roundtrip. Most companies have coupons on their websites, too.
Please note, none of the above prices includes tip. I will also say, that I took Mears once, and even if I was saving a couple of bucks (and I would be this time), I would still spend some extra for the convenience of the towncar.
Ciao,
Mort.