I used Pachy forends on octagons all the time from when first offered until I sold my last octagon a few years ago, and they work just fine on the octagons. In my case mostly with the custom hanger bars I used on all barrels rather than those from Pachy though (to get the forend off the barrel completely).
With different vintages of factory forends for them prices can vary widely. Some are more popular than others, some down right rare now days (like those that was a ball stud and snap on/off - the Herrett style getting scarce too). You might find one used for $10 or less all the way up to $80-$90 depending on which model, grain and condition.
With a hanger bar you can mount just about any forend on them, doesn't have to be one routed for octagon.
