Example of using intl to format input value.

input_formatted.dart

import 'package:flutter/material.dart';
import 'package:intl/intl.dart';

void main() {
  runApp(MyApp());
}

class MyApp extends StatelessWidget {
  // This widget is the root of your application.
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      theme: ThemeData(
        primarySwatch: Colors.blue,
        visualDensity: VisualDensity.adaptivePlatformDensity,
      ),
      home: MyHomePage(title: 'Flutter Demo Home Page'),
    );
  }
}

class MyHomePage extends StatefulWidget {
  MyHomePage({Key key, this.title}) : super(key: key);

  final String title;

  @override
  _MyHomePageState createState() => _MyHomePageState();
}

class _MyHomePageState extends State<MyHomePage> {
  int _inputValue = 0;
  var f = NumberFormat('###.00', 'pt_BR');

  @override
  Widget build(BuildContext context) {
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Padding(
          padding: const EdgeInsets.symmetric(horizontal: 10.0),
          child: Column(
            mainAxisAlignment: MainAxisAlignment.center,
            children: <Widget>[
              TextField(
                decoration: InputDecoration(
                  border: OutlineInputBorder(),
                  labelText: 'Value',
                ),
                onChanged: (value) {
                  setState(() {
                    _inputValue = int.parse(value);
                  });
                },
              ),
              Text(
                'Value:',
              ),
              Text(
                '${f.format(_inputValue)}',
                style: Theme.of(context).textTheme.headline4,
              ),
            ],
          ),
        ),
      ),
    );
  }
}