Costs: I used about $20 worth of my husband's wood, bought two boxes of drywall screws for $7 (of which I have many left over for other projects), and will spend $12-$15 for a can of Danish oil later on. I did use many of the mistake screws, where three-inch screws would have been too long. Not that I am above sawing off or filing down a too-long screw once in a while. About half of the wood in the chair is salvaged from the previous armchair and from our scrap pile. I still need two more small pieces to finish the arms at the back. I was thinking of reusing the flat springs from the old chair, but didn't have hardware on hand for attaching them securely, so I put 1x2 slats across instead.
The cushion and cover are from the old chair. I cut down the cushion with scissors and a steak knife, and then refit the cover using slipcovering methods: put the cover on inside out, pin it to fit, mark the lines that need to be sewn, then remove the cover and sew them. This particular cover was essentially a slipcover made from old upholstery fabric anyway, so no cording or anything to worry about. I will replace it when better fabric becomes available, and it is getting washed on my very next laundry day.
The chair is more square than it looks in the picture; the skew in the seat support evened out under the weight of the back, as I thought it might. I might make a pillow for the back, but it is comfortable enough without it. I do get the feeling of sliding off if I slouch, though. The children alternate between using it as a royal throne, and a base on which to build couch cushion houses. It is sturdy enough to stand up to them standing all over it.