The 60% saving in capital budgets was primarily the result of axing the Building Schools for the Future programme. Some schools that did qualify for a rebuild have faced long waits while a few flagship free schools were fast-tracked. In terms of capital costs, several free schools are private schools which came with their own premises, so would have been cheaper. However, others are brand new and require refurbishment - e.g. Bristol Free School cost £8 million approx but was built in an area where there are 300 surplus places and, I believe, millions spent on PFI rebuilds in surrounding schools.
You could also take free schools in the wider context of converter academies, as free schools are essentially academies. There was a miscalculation and subsequent £1 billion overspend which has prompted the changes to core funding. Some areas such as Cheshire East have suffered 'devasting cuts' since then.
While some free schools may have lower capital costs they still receive an initial start-up grant and if those schools are undersubscribed they are initially being subsidised. They may have a detrimental effect on other schools, especially if they are exacerbating a surplus in rural areas of declining population. Two half-full schools would have higher overheads yet may be unable to offer a full range of subjects.
I have some sympathy for parent-led proposals but not if they affect neighbouring, successful schools. Ex-private schools may fill gaps cheaply if there are shortages but could be divisive if they are faith schools, especially because there already is an alternative and now easier VA route. Some of the new free schools have had 'Needs to improve' Ofsted reports while schools such as the Maharishi school in Lancashire have repeatedly been judged to have infringed advertising standards (that one did not even enter pupils for SATs). So such schools may cost more to monitor and regulate in other ways.
It's early days but I think the National Audit Office and Ofsted will give us more information as to the value for money and effectiveness of free schools and academies in general. There has been little transparency from the DfE.