You don't need steel molds for plastic injection molding these days, a number of companies now use aluminium molds. These don't last nearly as long as steel ones but really do bring the price down a lot.
A lot of the advances in casting and molding are happening in the miniature wargaming field, so that could be a way to direct your search.
One company you may want to talk to is Renedra: http://www.renedra.co.uk/index.php
They produce a number of quite large scenery pieces in plastic, they are really highly regarded in the industry and perform contract work for a lot of other manufacturers.
It may also be worth considering a resin-plastic hybrid. A few years back the strength wasn't really there but these days, with the recent advances, there are a few mixes which could well be strong enough. Going down this route avoids injection molding all together and opens up spincasting, meaning vastly cheaper moulds and the possibility of much shorter production runs. One US-based company you may want to look at is TrollForged and their TrollCast material: http://trollforged.com/store/trollcast.html
As with Renedra I believe these guys will handle the production end for you, leaving you with just the design and costs.
Hope that helps some.