Are acoustic boards worth double the price....

How many boards would you need? I would do it anyway on the basis (a) the neighbours might change, (b) it could be a good selling point.and (c) if you don't, if there is noise disturbance in the future you may then regret it and get stressed..

Not convinced so called acoustic board works any better than decent plaster board. You need mass to stop sound.

"soundblock" plasterboard is about 30kg a sheet compared to 24kg a sheet for "normal" 12.5mm stuff ...

I used 2 sheets overlapping of normal 12.5mm, making sure underfloor and ceiling were well sealed in the process. Seemed to make a difference - or next door's kids have become remarkably quiet.

Quite. Either thicker than normal plasterboard, or a double layer.

I went the whole hog in the lounge, stud wall built away from the party wall, filled with heavy rockwool batts, covered with barium loaded rubber, two staggered layers of 12.5mm soundblock plasterboard suspended from resilient bars, acoustic sealant ... it did make a lot of difference ... but now I just tend to notice the noise coming through via other rooms, and I can't really afford to lose 6" in them all ...

Two layers is what I have always done in these situations.

That very much depends on how you hear things. If its structure borne, then probably no much use, but if its just through the e wall all sorts of things might make a difference, but sound isolation is one of those things where the law of diminishing returns applies. Short of suspending a room within a room with lots of absorptive stuff and triple glazed windows you will always hear something.

Brian

I'm butting up to a door casing, can't get away with doubling up, that was my first instinct.

I'd be surprised if anything the same sort of thickness as plasterboard would make much difference in practice - short of lead.