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.
Latham's do a 3" rod tube for ?3.29.
They don't charge for postage for orders over ?10. The service is the best I've ever known. If you phone your order through in the morning you'll get it the next day.... And they're really nice people too!