test.dart

return Scaffold(
      body: Stack(
        children: <Widget>[
          Align(
            alignment: Alignment(0.4, -0.6),
            child: Container(
              padding: EdgeInsets.symmetric(horizontal: 40.0),
              child: TextField(
                autofocus: true,
                onChanged: (str) {},
                autocorrect: false,
                decoration: InputDecoration(
                  prefixIcon: Icon(Icons.attach_money),
                  hintText: '0.00',
                ),
              ),
            ),
          ),
          Align(
            alignment: Alignment.bottomCenter,
            child: GridView.count(
              shrinkWrap: true,
              crossAxisCount: 3,
              childAspectRatio: 1.7,
              children: <Widget>[
                Container(
                  decoration: BoxDecoration(border: Border.all()),
                  child: Center(
                      child: Text(
                    "\$100",
                    style:
                        TextStyle(fontWeight: FontWeight.bold, fontSize: 20.0),
                  )),
                ),
                Container(
                  decoration: BoxDecoration(border: Border.all()),
                  child: Center(
                      child: Text(
                    "\$320",
                    style:
                        TextStyle(fontWeight: FontWeight.bold, fontSize: 20.0),
                  )),
                ),
                Container(
                  decoration: BoxDecoration(border: Border.all()),
                  child: Center(
                      child: Text(
                    "\$400",
                    style:
                        TextStyle(fontWeight: FontWeight.bold, fontSize: 20.0),
                  )),
                ),
                Container(
                  decoration: BoxDecoration(border: Border.all()),
                  child: Center(
                      child: Text(
                    "1",
                    style: TextStyle(fontSize: 20.0),
                  )),
                ),
                Container(
                  decoration: BoxDecoration(border: Border.all()),
                  child: Center(
                      child: Text(
                    "2",
                    style: TextStyle(fontSize: 20.0),
                  )),
                ),
                Container(
                  decoration: BoxDecoration(border: Border.all()),
                  child: Center(
                      child: Text(
                    "3",
                    style: TextStyle(fontSize: 20.0),
                  )),
                ),
                Container(
                  decoration: BoxDecoration(border: Border.all()),
                  child: Center(
                      child: Text(
                    "4",
                    style: TextStyle(fontSize: 20.0),
                  )),
                ),
                Container(
                  decoration: BoxDecoration(border: Border.all()),
                  child: Center(
                      child: Text(
                    "5",
                    style: TextStyle(fontSize: 20.0),
                  )),
                ),
                Container(
                  decoration: BoxDecoration(border: Border.all()),
                  child: Center(
                      child: Text(
                    "6",
                    style: TextStyle(fontSize: 20.0),
                  )),
                ),
                Container(
                  decoration: BoxDecoration(border: Border.all()),
                  child: Center(
                      child: Text(
                    "7",
                    style: TextStyle(fontSize: 20.0),
                  )),
                ),
                Container(
                  decoration: BoxDecoration(border: Border.all()),
                  child: Center(
                      child: Text(
                    "8",
                    style: TextStyle(fontSize: 20.0),
                  )),
                ),
                Container(
                  decoration: BoxDecoration(border: Border.all()),
                  child: Center(
                      child: Text(
                    "9",
                    style: TextStyle(fontSize: 20.0),
                  )),
                ),
                Container(
                  decoration: BoxDecoration(border: Border.all()),
                  child: Center(
                      child: Text(
                    "C",
                    style: TextStyle(fontSize: 20.0),
                  )),
                ),
                Container(
                  decoration: BoxDecoration(border: Border.all()),
                  child: Center(
                      child: Text(
                    "0",
                    style: TextStyle(fontSize: 20.0),
                  )),
                ),
                Container(
                  decoration: BoxDecoration(border: Border.all()),
                  child: Center(
                      child: Text(
                    "Done",
                    style: TextStyle(
                        fontSize: 20.0,
                        color: Colors.yellow[800],
                        fontWeight: FontWeight.bold),
                  )),
                )
              ],
            ),
          ),
        ],
      ),
    );