Ventilation ducting v soil pipe

Any idea why white vent pipe sized 100mm is so expensive?

I want to run some pipe for a bathroom extractor and would prefer to use a rigid pipe as opposed to the flexible stuff. I need to span 3.7m.

A 4m length of soil pipe will work well and cost £18.

A 1.5m length in 100mm ventilation pipe costs £16 on ebay. So thats £50 approx for the same job.

I will be boxing in so this wont be seen.

Seen soil pipe used by sparks, but if you did want vent ducting, why not use https://www.tlc-direct.co.uk/Main_I...x/Ducting_Round/index.html#4_Inch_Accessories x2 2m length and adapter?

Thanks this is much more reasonable.