Can you send cost for adding this feature to pm
There's no cost in adding features to Hercules, but it may take some time as I have to analyse our source-code first, and then test everything.
Another issue to recall is multi-platform support. It'd have to be platform specific implementations for linux and windows, both of which have their own optimal solutions (linux has POSIX threads, pthread.h, and windows just has all the functionality within windows.h to create threads, though I believe there's a pthread for win32 out there, I've never used it). Boost also has its own thread library, but that's C++, at which point you might as well use C++11 standard if you had that option for the built-in threading.
Though pan is right, the major issue is trying to figure out what you can really throw into threads safely. At the very least, it'd be interesting to see certain scripts be async. That is to say, if there are no global dependencies (only character variables being used in the script), then it might be totally feasible to handle processing in different threads. Imagine 1000 people using the refiner, and instead of each user blocking another (which isn't noticeable unless you have a high volume of players usually), each one runs asynchronously.
Though there's also the overhead of thread creation to consider.
Then there's also creating testbenches so you don't need 3000 players to test how well it worked.