As more of the contractors on the river do there sums and realise that timber has increased in price massively, prices are getting more job specific. You should be looking at a minimum of £135 per meter, that does not include back filling, and if it is any less, then theres a fair chance you'll be getting an inferior job.
Price is mostly dependant on mobilisation cost and the length and quality of the timbers used.
Wood isnt what it used to be, with many of the chemicals that preserved it removed and the pressure treatment not getting adequate penetration the lifespan is a lot shorter than it once was. contrary to what some contractors may tell you, they all use scandinavian timber. However some have started using red wood which does seem to be of a higher quality.
Steel is very expensive, and galvanised steel even more so but it does offer a considerably longer lifespan and better residual value to the plot or property. In the long run it probably represents better value for money these days.
Plastic is horrible nasty dreadful stuff and good luck finding a contractor who wants to put it in.
Alder pole piling doesnt represent very good value initially because of its short lifespan and because of how difficult it is to souurce in any kind of quantites, however i think it looks quite attractive and when combined with soft engineering (planting reeds) can be a good long term solution, as long as you dont have a boat you want to moor up.
Basically, make sure you get lots of quotes and be aware of what you want from the outset.
Most of all remember, anyone can talk a good job, so go and see some of there work for yourself.