Don't I wish? = Don't I wish! = If only!
These mean something like
If you or I believed that, we would be deluding ourselves. It's not at all true, though I wish it were true. It would be nice if it were true.
-- But why not buy that luxury car? You have plenty of money. You can afford it.
-- Don't I wish!
-- So what if that homework is due tomorrow, an