How expensive would it be to replace a manual transmission system to an automatic?
In most cars it's a major operation and quite expensive. There's usually body and mount modifications that need to be made. You'll need the new trans and all the controls. Some cars have a different ECU and wiring harness for each transmission. Possibly vacuum system modifications. Sometimes it even requires changing out drive/halfshafts, etc. What make/model do you want to do this too, and how much of the work can you do yourself?