Whats the most affordable way to setup a large screen outdoors for live video
"Most affordable" makes this a very difficult question to answer. The way most festivals do it is by hiring large screens. You could try and do it with a projector, which can work okay at night, but not so good in daylight (like a drive-in cinema). 

For bright displays there aren't really any shortcuts - 


Have a good video platform and desk
Route signal
Use good quality screens


Technology-wise, it is very simple, as you aren't doing anything clever with video.