Hi

Anyone know how much they are to buy? It's for a 9mm full size.

Also, do they offer much improvement

Thanks

They offer a huge improvement in feel, but not so much in actual pull weight.  You can get them from Cal's or OPS2 for about $120 or so, or HK can install them.  I would recommend having HK install the trigger because while many people drop them in with no problems, every single time I have heard of a problem with a match trigger install it has been from a drop in.  Let HK install it and set it up right and you will get all the trigger has to offer.  Which is, a very crisp trigger pull with no creep or overtravel, smoother DA pull, and a SA pull that feels lighter than it actually measures.  The pull on my USPs run about 4.75 pounds SA, while the pulls on the Tac/Expert/Elite run about the same, they feel much lighter due to no creep or OT.
Also, the match trigger kit can only be "dropped in" in pistols of the following serial numbers.
USP9F (SN: 24-21515 and higher)
USP40F (SN: 22-36242 and higher)
USP45F, all
so if your pistol's serial number is below the above the pistol MUST be returned to HK for the match trigger to be installed.
Cal's
OPS2

Thanks for the great info

Steve