I suspect the increased costs will also probably relate to all the things that would have been required through more detailed design and weren't necessarily shown on the outline drawings. These could have been significant.
I noticed that Killie have installed what looks like executive boxes at the back of the stand opposite the one we were in. Those looked like a much more cost effective and practical solution than what we were proposing. Killie's boxes would probably have utilised things like existing accesses/escapes, shared toilets, no foundations , no drainage , existing roof etc. Something similar could perhaps be done at the Family Stand with the loss of some seats to the rear half over part of the stand's width??