Cheapest Hallucinogen

Stay off the nutmeg, horrible side effects for days after....
Seriously if you need to look for cheap halucigens, than maybe it is not the right time in your life to dabble with these things.

The best stuff is free, well sure you need to grow it, and it takes time to get a bigger repertoire than just haveing shrooms available, but hell, its so worth it.

Regarding DXM/Robitussin:

Get a product with DXM (dextromethorphan) as the ONLY active ingredient. Guafenisin, chlorphenarmine maleate, and numerous other antihistamines and medications are present in commercial cough medicines and can cause unwanted side effects and even death. I recommend reading the DXM FAQ before taking this substance. It is available along with a host of other useful information on www.erowid.org

DXM or morning glory seeds. you have to get the right seeds though they will make you sick if you get the wrong ones