Price not too bad but you need to be careful because not just any old pipe will fit. My centre box replacement pipe cost me £40.00 from a guy who bought it for his V8 Disco but it wouldn't fit. Went straight on my RRC though. I believe the difference is at the Y pipe, there are 3 different ones (certainly for RRC) Manual, Auto and I think the other is Universal. So would suggest you check it out first. The other thing you need to bear in mind that a straight through pipe (as in no silencers) is unlikely to pass an MOT. Removing the centre box, your emissions will change again possibly failing MOT and lastly, if you are on a longer journey, the noise is likely to be tiresome if you don't have at least a standard rear box on and you'll wish you hadn't made it quite so loud. Of course it will sound great but you also have to be ready for more attention from plod if it is too loud.
Chaning the centre box for a pipe won't do any thing to the emissons it will make it louder and probably vibrate the floor a bit. Myadvice would be to replace the rear box with a straight through pipe.
From my own experience with this a straight through middle pipe makes for an 'angry' V8 sound but there is a tiresome, deafoning resonance from the backbox at 2000 - 2300 rpm, made worse by an automatic transmission because you often rev to that level and let the auto do the rest.
I currently have a centre silencer with a straight through rear pipe which is much quieter and sounds 'grumpy' rather than 'angry'.
Its pretty much rusted out now so I'm tempted to go for no middle silencer again, particularly as I have a manual 3.9 and can drive it through that resonance area quite easily.
Theres a man on fleabag who will supply me a cat back, straight through system (rear silencer only) for £92.00 inc VAT delivered.
Hmmmmm, I'm tempted
The standard centre box does produce a fair bit of back pressure, so removing it will mean that the mixture will need to be richer. I don't know which V8 you have, but that is something to consider.
We find that the standard system, when new, sounds nice and warbly
I once ran a 3.9 auto with a straight tail pipe after the rear box fell off (as they are prone to doing) & found it produced more waffle but was not really any louder, so just right! Was thinking of getting a bespoke pipe made up as it really needs a slight bend to prevent it contacting the bottom edge of the bumper. Can't comment on emissions though as a new system was fitted prior to the next MOT.