Remove the quotes from

 var long2 = double.parse('$group1');

to

 var long2 = double.parse($group1);

You can also use

var long2 = double.tryParse($group1);

or to also accept numbers without fractions

var long2 = num.tryParse($group1)?.toDouble();

to not get an exception when the string in $group1 can not be converted to a valid double.

Double To String

productprice.toString().

String To Double

double.parse(productprice.toString())

Please refer to below code

code snippet

For more info on toStringAsFixed. Please refer to this link description

  String num = "19";
  double val = double.parse(num);
    print('Result: ${val.toStringAsFixed(2)}');

Output:
Result: 19.00