Haste vs new weapon

[Edmonde Dantes]

This may be a stupid question, but I feel that I must be missing something. So, my main is a 20 monk with a wu's 2hb, and I am trying to plan on what to save up for next. I figure the two best options are peacebringer or fbss.

What I don't understand is that, after searching the forums, it seems the consensus is to get haste as quickly as possible. The reason this does not seem to make sense is that I have seen peacebringers for 2k (1800 once) and fbss for more like 5k, and yet when i do the numbers it seems like getting the peacebringer first is the better option. fbss/wu's would be better than peacebringer without fbss, but not by enough to make it worth more than double the cost. Where am I going wrong? I know that I will eventually get both, but at the rate that I make cash the decision matters to me.