Thats a good question. I wouldn't think true aquatic plants would be so much more expensive.
I would be interested to read more about this. Do you have any links?Mondo Grass (Dragon's Beard) is not a true aquatic plant, but in the right conditions it is able to thrive underwater for a very long time. Many people find that after only a month or two it begins to die off, this is quite often the case. Other times it readily adapts and thrives, shoots from this plant would be more viable underwater, particularly if well developed in it's original environment before replanting elsewhere. Experienced keepers successful with Dragon's Beard are very proud and find it is a very versatile decoration after it has grown strong underwater.
Success adapting Dragon's Beard to an aquarium is not to be expected but is something to be revelled in and enjoyed for a long time if it is achieved. Give it a shot, but I wouldn't mess with it too much while it is trying to establish. Consistency is the key during this period.