WHY CGI COSTS MORE:
cgi is more expensive because they need like 30 people to animate things then spend literally weeks rendering it out.
Firstly, high production films these days require multiple visual effects houses working on the hundreds of visual effects shots over a period of six plus months. "Post production" starts during principle photography, as soon as the shots are finished filming, since there simply isn't enough time to do it after.
A single visual effects shot is broken down into many different stages.
A visual effects supervisor on set needs to coordinate all the elements that will be required by the visual effects house, ie. Green screen, wire rigs, iMocap suits, etc.
Then the elements in the shots gets created, which comprises of sketching, modeling, texturing, rigging, shading, lighting and animating. All of which is done by separate departments within the visual effects house.
Animating usually consists of bringing in body actors in a specialized Mocap stage and performing all the movements the characters in the scene needs to do and feeding that information into the low poly model of the character.
In the case of iMoCap being used on set, the 3D model still has to be attached to the actors performance.
The shot needs to go through 3D tracking, wire removal, compositing of 3D elements into the scene, colour grading. This is also done with TEAMS of people, not just a dude sitting at a computer.
After which the shot needs to go through rendering. This is done with a horribly expensive Render farm and takes a decent amout to render every frame.
After which it needs to be viewed by the supervisors and directors. Changes WILL be made, so unless the element on-screen that needs to be changed is coming from a separate pass, it will probably have to be rerendered.
CGI requires A LOT of expensive people working with A LOT of expensive hardware for A LOT of time, Same reason games have huge budgets
That all adds to the bottom line of the contract that is signed with the many visual effects houses. CGI requires A LOT of attention to detail. That means a lot of people need to get paid
what precedent is there for them to make a choice like that 
