Okay, you need to do the FULL math job here, but I'll give you a sample.
For starters,
your rental car expense will be $588 plus gas and tolls, assuming your Costco quote included all taxes.
Per Ride Guru, for one day at Magic Kingdom, with a mid-day trip back to your resort and then back to MK, and back to your resort at the end of the day:
Without boring you with the details, the four rides on Uber X (max 4 people), comes to about $64 without tip. For the purposes of this exercise, forget tips.
Trips to DHS and Epcot might be a little less, AK might be a little more. It's also possible that you would not want to go back to your resort every day (although if you have a small child, those trips are very wise). If you skip the mid-day trips a few days, you lower the cost somewhat.
But for the sake of argument, let's assume you to two round trips per day, do
NOT tip

, and it all equals out in the end. So for 10 days you spend about
$640 with Uber.
But you still have three missing pieces to your transportation puzzle:
- Your trips on the two days you don't do parks
- Your non-park trips (DS, dinner, shopping, etc)
- And your transfers back and forth to the airport
The airport transfers are about $23-$25 each,
which puts us at about $700 using Uber -- not including any tips for your drivers,
and you still have the non-park days and rides to account for.
You should do your own math, but for a 12 day trip staying offsite I personally wouldn't want to be without a car. And I think the rental car will work out less expensive.