I have just the opposite problem ... my husband keeps telling me "You need a Long Arm" and I keep telling him all the reasons I don't want a long arm!!
As for cost justification, Dunster laid it out pretty well and a lot will depend on how much the setup you are looking at will cost and the number and size of quilts that you make. And like Lori stated there is the additional intangible cost of learning to use it properly (and all the thread, muslin, and batting you will go through).
For me there is one more pretty magnificent that I'm not willing to spend. Space. The long arm itself doesn't take up much room but the frame does ... and if I'm going to invest in a frame I wouldn't waste money on a frame that couldn't manage a King size so we're talking 12' long by 5' wide (minimum). My house is an ample size, but it's filled
I would however consider a mid arm machine that I can drop into a table, preferably my cutting table. When we are done redecorating the family room (almost done!), I'm moving my sewing room from one room into another and I want a cutting table made-to-order. I think I'll look into having it made with a hole for a mid arm, and a cover that will sit flush then my large cutting mat sitting on top. My "cost justification" ... mid-arm $4,000, made to order table $600, shutting husband up ... priceless!!