HI Dave,
Right off the top of my head, I would say roughly £300 (but this depends on particular decisions in materials / electronics / hot end etc). The so called "vitamins" of the bars, studding, fasteners etc becomes negligible once you buy a couple of machines worth (£30 ish). The expensive bits are the stepper motors ( required at around £12 each), the electronics (which I have bought in kit for for £30 + £30 for stepper drivers... no reason why we cannot buy bare board then source components ourselves, I do no since I am not electronically confident enough to do so!), then the hot end which has a couple of choices globally, 1 in the UK, 2 in Europe and 1 in the US (that are worth owning and are tried and tested). Again... no reason we could not build our own if we had access to a lathe, but this will require testing, development and most likely a couple of iterations in machining / design.
I just realised I've hit you with a wall of text. I reckon £300 is a decent figure, I would expect to be able to purchase filament with that as well (and would look to reduce the cost beyond this if possible).
Cheers,
Iain