Personally, I think you should charge for everything. I played Confusion as a free version (I think it finished around chapter 5 or 6) and then paid the $12(?) for the full version a month or so ago and played through it in its entirety; well worth the few dollars.
Consequently, have just paid for the pre-order. If people aren't prepared to pay a pitiful $3 to sample the game then they shouldn't get anything for free.