This may give you a rough idea at what you could be expected to pay:

I bought some steel a few weeks back, a couple of 100X100x4.0
Rectangular Hollow sections cut to around 7.5 ft in length and paid
£104 each.

I also bought a larger RHS 200X100x6.0 which was 3.3 metres long
(£180). This thing weighed a ton and me thinking I could of done this
myself!! it took 4 of us to lift into place resting ontop of the
other 100x100x4.0 sections. Ask a neighbour to help you out or recall
some favours from your mates. If your just dragging it and not lifting
over head you should be fine with one person helping you out I would
think.

Mmm... OK, cheers for all the comments. Well, based on shop4steel.com it
was going to cost £518.25 for 3 x 12.2m lengths of 200 x 75 PFC, but that's
going to be completely impractical. I've phoned around a few steel
stockists and they're after ~£800 for the 12 lengths cut and pre-drilled.
Just waiting on one more company to get back to me.

Apart from anything else, I'm slightly concerned about the ceiling taking
the weight of all this steel while I'm manoeuvring it into position. In
case you hadn't guessed, I've had the drawings back from the structural
engineer and I need to bolt PFC to either side of the two big chords running
from the front to back of the loft
(http://www.pciq.co.uk/pics/loftplan.gif). Think I might have to use some
temporary props to support the ceiling while I'm working on this, especially
if I'm going to have a few people up there helping me...

One final question, how realistic is it that I'll be able to drill 20mm
holes in this stuff myself? Not at all I suspect... For the sake of an
extra £150 or so I think I'll opt for pre-drilling.

I can see this being fun... :-)

Andy