A relative simple way to get the instance object from a custom property drawer that draws a serialized class inside a component

Example.cs

using UnityEngine;
using System.Collections;

public class Example : MonoBehaviour
{
	public ExampleClass example;

	void Start()
	{
		
	}

	void Update()
	{
		
	}
}

ExampleClass.cs

using UnityEngine;
using System;

[Serializable]
public class ExampleClass
{
	public string field = "example string";

	public void Print()
	{
		Debug.Log("ExampleClass.field: " + field);
	}
}

ExampleProperty.cs

using UnityEngine;
using UnityEditor;

[CustomPropertyDrawer (typeof(ExampleClass))]
public class ExampleProperty : NestablePropertyDrawer
{
	protected new ExampleClass propertyObject { get { return (ExampleClass)base.propertyObject; } }
	private SerializedProperty stringField = null;

	protected override void Initialize(SerializedProperty prop)
	{
		base.Initialize(prop);

		if (stringField == null)
			stringField = prop.FindPropertyRelative("field");
	}

	public override void OnGUI(Rect position, SerializedProperty prop, GUIContent label)
	{
		base.OnGUI(position, prop, label);

		EditorGUI.BeginProperty(position, label, prop);
		EditorGUI.BeginChangeCheck();
		EditorGUI.PropertyField(position, stringField);
		if (EditorGUI.EndChangeCheck())
		{
			stringField.serializedObject.ApplyModifiedProperties();
			propertyObject.Print();
		}
		EditorGUI.EndProperty();
	}
}

NestablePropertyDrawer.cs

using UnityEngine;
using UnityEditor;
using Object = System.Object;

public class NestablePropertyDrawer : PropertyDrawer
{
	protected Object propertyObject = null;

	protected virtual void Initialize(SerializedProperty prop)
	{
		if (propertyObject == null)
		{
			string[] path = prop.propertyPath.Split('.');
			propertyObject = prop.serializedObject.targetObject;
			foreach (string pathNode in path)
			{
				propertyObject = propertyObject.GetType().GetField(pathNode).GetValue(propertyObject);
			}
		}
	}

	public override void OnGUI(Rect position, SerializedProperty prop, GUIContent label)
	{
		Initialize(prop);
	}
}