There is no objective way to decide at what price point it is worth it to buy a new technology. Everyone has to make that decision for themselves.
Economists say that we base our buying decisions on the amount of utility we expect to get from a given purchase. When the amount of expected utility is greater than the price of the good or service, we buy. The problem with this is that it cannot predict the behavior of any individual consumer. We all have different utilities that we assign to a given product at a given time.
If someone is an early adopter, they assign great utility to being among the first to buy a new technology. They get a lot of psychological satisfaction from this. Therefore, they will be willing to buy the product when it is still quite expensive.
I, personally, am not an early adopter. I tend to wait until the price of a new technology drops because I do not get much utility from being among the first to buy a new gadget. However, this does not mean that I have some formula for knowing when the price is low enough. It is just something that I (like everyone else) decide on gut instinct. There is a time when the price of something like an e-reader feels like it is low enough and I am willing to buy it.