I would argue that HD only subs pay far more of a "tax" then sd+hd subscribers do. Take a look at this post where I break down the costs:
Turbo Value
Basically, the value for HD only packages are horrible. For a minimal nominal increase in price to say switch from Turbo Bronze to Bronze 100 w/HD, you get back an overwhelming value.
Us Turbo subs pay a heavy premium to basically have a bunch of SD channels locked out for us; I am thinking I may be better served by switching to a regular package and then doing the locking myself...