What can you expect (in terms of return percentages) when you play blackjack by the basic strategy? It depends on the game you're playing; the number of decks being dealt and the rules in force affect the expected return. For almost all games you encounter, the basic strategy player's expectation is between -0.7% and +0.1%. The majority will actually be between -0.6% and 0.0%.

We've generated the following table to guide you in estimating the approximate effects of rules variations on the basic strategy player. (The table was generated by simulation assuming our generic basic strategy and should be used only as a guide. The effects of rules variations often depend on the number of decks in play. We have assumed the average effect for the purposes of constructing the table on pg. 37.)

The benchmark is for the set of rules often referred to as the "Las Vegas Strip game." The top-of-the-deck expectation is effectively -0.02%, making it nearly an even game.
To estimate the expectation for a particular game, we can, to a good approximation, start with the benchmark and adjust for any rules changes by simply adding their effects.