Calling From The Big Blind

[London Pat]

I would be grateful for people's thoughts on when it is appropriate to call from the big blind with trash hands. Here's a simplified example:

Blinds are 100-200. Assume all stacks are average at about T8000 (i.e. well over 20xBB - think the second hour of an online rebuy tourney) and you have no knowledge of the other players involved.

MP raises to 600 and gets two LP callers.
If you are in the big blind with 84o, it costs 400 to call into a pot of 2100, odds of approximately 5/1. Should you make this call?

Regardless of the answer to this specific situation, the obvious next question is what odds would you need to call with any two cards? And finally, what if your stack size was lower (say, T3000), would that affect the decision?

Thanks for your help. This is a common situation in which I often feel I have made the wrong decision.