We didnt have to pay when we went, its like shopping restaurants etc so they cant really charge you....can they!? lol. You can walk to universal city walk but i wouldnt from stop 29!! lol. If you use the i ride trolley & get of just by wet & wild there is a road & you can see universal from there, we walked back the one day takes about 10 - 15 minutes! Not sure how much a taxi would be, you could always walk there & taxi back?

Downtown disney we just got of on the lynx 50 near disney its the first stop as you come in to disney, if you ask the driver he will tell you, there are 2 stops one round back & one at front, not sure how much a taxi would be, we found the bus fine!

There's info on city walk on this link http://www.universalorlando.com/citywalk.html

And this is what Ive found out in my "planning" for our trip in a few weeks

CityWalk has no charge for entrance, but individual venue cover charges apply.
They also do meal deals where you get a party pass and a movie, or if you have an orlando flex ticket plus (not sure if its on the normal flex ticket) it also comes on the 2 park unlimited admission ticket you get a citywalk pass for free
CityWalk Operating Hours: 11:00 a.m. - 2:00 a.m.
Parking is daily rate of $12 or $3 after 6pm

Hope that helps