Re. cost of home birth vs hospital birth, IIRC then the Payment by Results charges were updated recently (earlier this year?) such that a PCT get the same amount regardless of place of delivery. My understanding is that these charge schedules were derived from average costs encountered - which suggests that for a straightforward birth, cost is not influenced significantly by location.
Re. unplanned vs. planned homebirths in the recent study - I think this was one of the big points of contention. In addition to the precipitous labours, "unplanned homebirth" also included those women who had declined all antenatal care for some reason (a sad one that sticks in my mind was that some were in denial about their pregnancy). Anticipated outcomes for these women would be expected to be poorer than for those that had a planned homebirth. (currently failing to find the ref for this, but I'm pretty sure it was made by a number of groups when that study was initially published, including the NCT)
I was leaning towards homebirth throughout my pregnancy, but DH took some more persuading. To me, the hospital was not a place where I felt safe. It wasn't a place where I felt cared for. It was a place where I felt out of control and scared by the noise of other women screaming.
In the end DH came round and I had a homebirth with DD. She was my first and breech. For us it wasn't a trivial decision - at the time there was no evidence to suggest CS gave better outcomes for either mother or baby over vaginal birth (the 2000 study being badly flawed), but everyone agreed that a vaginal birth with attendents lacking in experience was a more dangerous option. To guarantee breech experience we used IMs. IMs are rarely able to operate in NHS hospitals as MWs. So we had a choice of a CS or a homebirth. Was it an unnecessary risk? Not to us - we did the best risk/benefit analysis we could on the basis of the information available to us and took the option that seemed to have the best risk profile at the time (not made easier as few of the risks are directly comparable).
Our decision would not have been actively supported by the NHS, but they have no legal right to tell me I "cannot" do this. As has been said, none of the options are risk free - whether you're talking breech birth, VBAC, gestational diabetes or home vs. hospital, individually or in combination - there are risks and benefits to every course of action. Once maternal mental health starts being brought into play (and lets not forget that until obesity reared its head suicide due to post natal depression was one of the biggest, if not the biggest, causes of maternal death in the UK), one size doesn't fit all and maternal preference does become important.