There isn't a comparison on any amount of DXM you will be planning on taking.. on a really really high dose you can make a few comparisons, but it's not something you can throw in the same class of hallucinogens.

Buy Robitussin brand syrup (if you end up buying syrup that is), as it tastes the best. If you buy pills, walgreens has DXM (and guaifenesin) pills that are pretty diesel.

Don't get impatient after dosing, big doses of DXM can take over an hour to hit.