A form field that it's value is calculated based on the values of two others.

calculated_textformfield.dart

import 'package:flutter/material.dart';

void main() => runApp(MyApp());

class MyApp extends StatelessWidget {
  @override
  Widget build(BuildContext context) {
    return MaterialApp(
      title: 'Flutter Demo',
      debugShowCheckedModeBanner: false,
      theme: ThemeData(
        primarySwatch: Colors.blue,
      ),
      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> {
  String _amount = '0';
  var _assetQtyControler = TextEditingController(text: '0');
  var _unitCostControler = TextEditingController(text: '0');
  var _amountControler = TextEditingController();

  void _updateAmount(_) {
    double dQty = double.tryParse(_assetQtyControler.text);
    double dCost = dQty == null
        ? null
        : double.tryParse(
            _unitCostControler.text,
          );
    String amount;

    if (dCost == null)
      amount = '0';
    else
      amount = (dQty * dCost).toString();

    setState(() => _amount = amount);
  }

  @override
  void dispose() {
    _assetQtyControler.dispose();
    _unitCostControler.dispose();
    _amountControler.dispose();
    super.dispose();
  }

  @override
  Widget build(BuildContext context) {
    _amountControler.text = _amount;
    return Scaffold(
      appBar: AppBar(
        title: Text(widget.title),
      ),
      body: Center(
        child: Column(
          children: <Widget>[
            TextFormField(
              controller: _assetQtyControler,
              keyboardType: TextInputType.number,
              decoration: InputDecoration(labelText: 'Asset Quantity'),
              onChanged: _updateAmount,
            ),
            SizedBox(height: 20),
            TextFormField(
              controller: _unitCostControler,
              keyboardType: TextInputType.number,
              decoration: InputDecoration(
                labelText: 'Unit Cost',
                prefixText: 'Shs.',
              ),
              onChanged: _updateAmount,
            ),
            SizedBox(height: 20),
            TextFormField(
              controller: _amountControler,
              decoration: InputDecoration(
                labelText: 'Amount',
                prefixText: 'Shs.',
              ),
              enabled: false,
            ),
          ],
        ),
      ),
    );
  }
}