I dont call about ?3.00 for a rod tube expensive, its cheaper than the drainage pipe. Drainage pipes are made from High Impact plastic, and do crack. I know cos I tried one, the careful airport luggage handlers managed to crack the bloody thing. I recently purchased a Carp Rod from WMAC, they sent it in a really good polythene tube with end caps, about 4" Dia, at no charge.I would strongly suggest DO NOT use plastic, if you must go to a builders merchants get Polypipe, with some end stops, more expensive than similar rod tubes,as supplied by Tackle shops, but wont crack.
Alternativley go on E-Bay, type in Rod Tubes, and loads of Cordura Tubes come up, again at quite cheap prices.