Unless you're doing other non-Disney things, I think a shuttle or towncar would be the cheaper route. For the number of people you have, it looks like Mears would be close to $70. For just a little more (I think going rate is $80-$90) you can get a town car all to yourselves, and I think Tiffany offers a free 30 min. grocery stop. Also, check the website to see if they have any online coupons.
I know this is a budget trip for you, maybe what you could do is use the grocery stop to get some snacks and drinks, and maybe a few simple breakfast items, which will save you money in the long run?