I guess what you want to hear is this:
Training D gives one more sound per pound, so to speak. Maybe his value would be the same as if you trained O, but his salary will be lower. and this will make him more valuable.
A buyer will think: Oh, good player and with such low salary!
What is more important though is there are much fewer balanced players, so they tend to cost more.
p.s. Don't forget that IS doesn't increase SF's salary.