My rule of thumb has always been...from an established author, I would pay up to £10.00 for a paperback, depending on the sample and the size of the book.
For both an established and self published author, I generally don't want to spend more than £3.00 on an eBook, just because I can not justify the reason for the high price for this type of book.
However, if I have paid £3.00 for an eBook and there are problems with the spelling, grammar and overall story, even though it is not a lot of money, I still get annoyed that I paid for something that some one couldn't be bothered to check through before releasing it! (no offence but this is usually with self published authors and I am not slamming anyone as I am also a self published author but I use a proof reader before publishing my work as these little things can destroy a great read!)
OK rant over! lol
PS - If anyone has read any good Chick Lit novels recently, please let me know as I need a decent holiday read for next month and I'm coming up blank at the moment!