In interpreting the info in the link provided by Alphatron, it seems that they go about 30 to the tonne (no mention of section or length), compared to hardwood (std gauge) of about 12/tonne, steel 20/tonne and concrete 4/tonne. It seems a lot of their field testing has been in QLD, maybe that 30/tonne is for 1067 gauge sleepers.

The Duratrack/Integrated Recycling website states that 1 tonne of recycled plastic feedstock makes 30 sleepers. That doesn't include the binders, fillers and other additives that are required to make the sleepers meet their rigidity, density and UV stabilisation specifications, so they will be heavier than 30/tonne. My understanding is that they have a comparable weight to redgum & grey box timber sleepers and the costs are on par with concrete sleepers - but more expensive than new A grade river redgum sleepers, even considering the difficulties in obtaining good redgum these days. This is the reason why their marketing is exclusively about comparing them to timber sleepers (not concrete) and explicitly does not mention sleeper-for-sleeper costs and instead promotes their superior lifespan.

There are competing plastic/composite sleepers on the market that are made overseas and built for heavier duty applications and are thus supposedly equivalent to concrete. It remains to be seen whether their trials will receive the fanfare that Integrated Recycling's product has gotten so far.