Steve
I don't know how much use this is going to be and it may even be too late for your deadline.
In my experience, a sparkle tends to work out at about 50% of the cost of the builders, whatever that turns out to be but we have never done a multi storey car park so I struggle to be any more specific than this.
As somebody else has already mentioned, it is very difficult to price these type of jobs if you don't have a relationship with the site manager as each one has different ideas of when the cleaners should be called in. Another complication is that despite assurances that the site will be empty of other tradesmen, you invariably find yourself working around painters, sparks etc which is not insurmountable but it does slow things down.
Not a lot but hope it helps
Mick