First: You should really read the FAQs on how to post a question and what information should be added. (No, I'm not the downvoter)
Here is an example how to get your mapping to work. Please note, that I've changed your classes a bit, because AutoMapper needs properties.
Source source = new Source();
source.Id = Guid.NewGuid();
source.Price = 10.0;
Mapper.Initialize(x => x.CreateMap<Source, Destination>()
.ForMember(a => a.Difference,
b => b.MapFrom(s => new DestinationDifference() { Amount = (decimal)s.Price })));
Destination destination = Mapper.Map<Source, Destination>(source);
Classes:
class Source
{
public Guid Id { get; set; }
public double Price { get; set; }
}
class Destination
{
public Guid Id { get; set; }
public DestinationDifference Difference { get; set; }
}
class DestinationDifference
{
public decimal Amount { get; set; }
}
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