You want to hire an instructor for a day for private tuition in advanced driving, going rate £500 - £800 per day based on experience and facilities needed. Does that answer the question?
Most instructors are happy to give their time back to RoSPA or IAM free of charge to help others learn the art of advanced driving. A lot of the fee goes on the materials you get and even if it didn't you get about 40 hours of expert professional tuition which works out at about £2.83 per hour as opposed to £80 per hour pretty good value in my book.
With regards to the standard between IAM and RoSPA, the basic IAM test is similar to RoSPA Bronze standard. This is the elementary standard but is a good starting point before you move to RoSPA silver and gold or the advanced IAM exam.
Since both are run by volunteers, I would choose the organisation with the local group who can best support you to pass your exam and also improve other aspects of driving as well as social events.
May I suggest you go along to the local group meetings and get a feel of how they run things in your local IAM or RoSPA group?
In London, we have tailored two observation/coaching sessions - one for those who can only attend during weekday in which we allocate an observer to them on a 1 to 1 basis; and we also run Sunday sessions twice per month for those who can only attend at the weekends.
In addition, we also organised other events which as country drive days for us London drivers who don't have enough country roads experiences, skid-control training, treasure hunts, quiz nights, group nights gatherings with presentation on anything automobile related topics such as current technologies, open forum discussion with IAM and police examiners, etc.
Speaking from personal experience, I have passed IAM 2 years ago and have now moved on to the committee and have become a observer. Since then I considered completing the RoSPA gold standard but have instead joined the HPC and went down that route.
At the moment I am hoping to become a senior observer soon as well as taking the advanced IAM test which requires Police Class 1 standard.
Please PM me if you have any questions.