Supposedly (according to the product literature), a dose of Deplin provides many times the usable folate that a dose of folic acid provides. They say this is because the body must slowly break down folic acid into folate, whereas Deplin is in a form your body can use right away.
I'm paraphrasing, but that was the gist of it.
I agree, you'd think it would be covered by insurance...the ins. companies have an excuse for everything it seems...