Having an actual decimal value as parameter for an attribute (example xUnit.net's [InlineData]

You should be able use the String value in the Attribute and set the Parameter type to Decimal, it get's converted automatically by the Test Framework as far as I can tell.

[Theory]
[InlineData("37.60")]
public void MyDecimalTest(Decimal number)
{
    Assert.Equal(number, 37.60M);
}

If this doesn't work then you can manually convert it by passing in a String parameter.

[Theory]
[InlineData("37.60")]
public void MyDecimalTest(String number)
{
    var d = Convert.ToDecimal(number);
    Assert.Equal(d, 37.60M);
}

Instead of InlineData, use MemberData as shown here. This gives you much greater flexibility in setting up multiple tests and allows the use of decimals or any other non-constant type.

public class CalculatorTests  
{

    public static IEnumerable<object[]> Data =>
        new List<object[]>
        {
            new object[] { 1.2M, 2.1M, 3.3M },
            new object[] { -4.000M, -6.123M, -10.123M }
        };

    [Theory]
    [MemberData(nameof(Data))]
    public void CanAddTheoryMemberDataProperty(decimal value1, decimal value2, decimal expected)
    {
        var calculator = new Calculator();

        var result = calculator.Add(value1, value2);

        Assert.Equal(expected, result);
    }
}