This is a bit of a "how long is a piece of string" question because it really depends on your requirement - eg hard shell or soft, whether you want one big compartment or several smaller, etc.

I prefer a decent-but-not-fancy squishy nylon bag - of the type often sold in the middle of Aldi or Lidl for £10-£15 - and a fancy hard case for the laptop inside it, everything else being either not vulnerable, or wrapped in my spare shirt. Squishy bags can be loaded (or not) to the demands of the situation and airline.