Total career salary is a good starting point. Sure, all $30M-career players are not created equal, but it's a good start. I'm sure everyone does this a little bit differently. I typically weigh a few things:
-- positional scarcity (I value C, 2B, SS, 3B are more highly than 1B/OF of similar quality)
-- Career value (total career salary is a decent proxy for this, but there are other methods)
-- Peak value (most of the time, I'd rather have 2 awesome and 2 mediocre seasons than 4 good ones, all else being equal)
-- How well peak value matches up with the peaks of other players on the team
Unlike a lot of owners, I do not believe that pitching always trumps offense in WIS, especially in progressives. If it fits the needs of my team, I'll trade an SP for a position player of comparable peak and career value, especially if the position player is a catcher or middle infielder.
Also, because I am sure everyone uses a different method for valuing careers and trades, it drives me nuts when owners in a progressive veto a trade. Unless the trade is just over-the-top blatantly an example of tanking or favoritism (eg: trading a star player for a scrub or a AAA, with no draft picks involved), I don't think trades in progressives should ever be vetoed.