"cheap" and "expensive" is a matter of taste most of the time. Other times they are very clearly defined, as in the price of stocks. Stocks are considered "cheap" if they appreciate and pay out enough dividends in less than 6 years to recover the capital expenditure. 6 to 10 years is fair, and anything above that is considered expensive.
With the price of a laptop, "cheap" can be defined in terms of
- Whether you believe you would be willing to pay more for the specifications and features
- Whether it costs less than similarly spec'ed laptops.
- Whether you should be able to use the laptop in a way that you could recover the cost quite easily (like doing development or design work).
- Whether the amount of work you'd have to do to save the money to buy it is relatively little. For instance, I view working five days to pay for a laptop as good value, but six days aren't. But that's my subjective opinion on it.
What I'm getting at is that you first need to decide WHAT you mean by "cheap" before we can give a good answer to it. For instance, many people think 99c for a certain piece of candy is cheap, but $1 is expensive, so it's very much a matter of cognitive dissonance in that case.
In the case of hot and cold, anything that can cause burns is considered hot, and anything that can cause pain is considered cold. these temeratures can be pinpointed scientifically. Everything in-between is a gradient.
It really is a matter of what the situation is.