indeed. there are many other factors to add into price other than just "accuracy" or simple "my gat's better than yours" one-up-manship.
the bit of pipe sticking out the end makes its own little difference to the marker - and not in terms of accuracy. as pete says, they barrels make little difference in themselves. over the range at which paint is usually traded the difference is negligible - if - and here's the if - the paint is consistent.
consistent paint which fits your barrel will be the biggest determining factor in barrel accuracy. there is no single barrel that will be better than any other single barrel. a cheapass barrel will be perfectly accurate if it matches your paint perfectly.
and the other if - the marker needs to be consistently firing at its velocity. at shorter ranges a fluctuation will make little difference - thus your £100 marker can accurately hit a paintball at 15ft - as the initial flight path is still relatively flat. the difference in accuracy at range is caused by the drop in the flight path - the slower the forward velocity the more pronounced that drop becomes. thus a marker with a wide swing in velocity will be accurate at close range, but less so at longer range.
at even longer range the effect is even greater, and no matter how "good" your marker, your accuracy will go out the window.
but this is getting off the "barrel" question.
what many manufacturers try to do is, in effect, gimmicky. some of it works, but not because the barrel is more accurate in and of itself, but becasue it fits your paint better.
they try to do this with barrels that change bore size at different points, or with different sized back/front bore sections.
then they throw in nonsense like "rifling". problem with rifling a paintball is that the inside is a liquid (no sh!t...!) so the outside case doesn't spin the same as the inside - completely negating any point in spinning the ball!
the best person to explain this in my opinion is Ryan from The Clan. He has a degree in aerodynamics/aeronautics/aerosmith (something like that!) and nows his shizzle.