Mike Salfino breaks down expected RBI totals for players, given their total bases and home runs.

The Bill James' formula he cites amounts to: Total Bases divided by four, plus home runs. Of course, that's assuming an average amount of RBI opportunities, and where you hit in the lineup and also the on-base skills of the players ahead of you will in large part determine that.

One thing Salfino doesn't mention - and I think should be considered when discussing RBI - is walks. Because the only time a walk is an RBI is when the bases are loaded, and even then, one RBI is not the ideal outcome. So players who walk a lot get fewer at-bats, fewer total bases and less RBI. (I suppose it could be argued that players walk rather than make outs - because they're not swinging at bad pitches, but that would only hold if players walked on four straight pitches. If you're taking strikes, you're losing RBI opportunities).