As a hybrid technology, Flutter automatically gains the cost advantage, when compared with native technologies, that only one code base will be necessary.
In comparison to some of the other webview or hybrid technologies, Flutter provides plugins for the device APIs which works on both iOS and Android based on the same implementation.
Based on my own evaluation, I would argue that Flutter could be a candidate for the quickest hybrid technology in the development phase. (Hot reload without state loss and minimal OS device API integration)
However, since Dart, the programming language of Flutter, is a relatively new language, which not a lot of developers have adopted yet, it is difficult to say if you will be able to save anything on the development cost. Supply and demand of Flutter developers needs to be stable before that can be determined.