Ok, so I had a reply from Patrick Motorsports and the conversion parts from them came out to $3670 broken down as below:
$40 Engine case to g'box studs
$285 Mount adaptor
$90 ARB/Sway bar adaptor
$690 Hydraulic conversion kit inc reservoir, brackets, hoses, sleeves, clutch m/c, slave cyl, lines etc
$225 Shift rod adaptor
$650 Flywheel
$22.50 Flywheel bolts
$1400 Clutch set
$245 Starter ring gear
$22.50 Bolts
They also sell the guide tube for $130 - lots of info here:
Transmission > Porsche G50 SBH (Short Bell Housing) By Patrick Motorsports Porsche & Mid Engine Performance Specialists
I alsodiscussed it with an engine & gearbox specialist in the UK who thought the modification work to the the gearbox would probably be around £2000 / $3150. This is to deliver him a gearbox and for him to strip it, carry out the machiing work and rebuild it. Anything worn or broken whilst in there would obviously be extra.
So as a quick tott up for the assumed total:
$3670 Patrick Motorsports conversion parts
$3150 Gearbox modification
$4000 (approx) for G50 Gearbox
$1000 (approx) Pedals, shift rod & sundries
It looks to be approximately $12,000 / £7700 for the conversion + a boat load of work + the unknown theoretical expense of a used G50.
all of a sudden I'm developing a new love for my 915! I simply couldn't justify that sort of expense. On the plus side it does give a new justification for dropping some coin into a killer 915 build
Hopefully this is of some use to anyone else considering the same conversion.