Take a publico (shared taxi running along fixed route) or the San Juan-Pto Plata guagua (van type bus) to La Javilla in Puerto Plata. It will stop outside the Javilla Tours Bus Station. There catch the guagua (bus) towards Santiago. Ask the cobrador (fares collector) to drop you off at Damajagua. Both services are fast, frequent and very cheap. Total fare one way less than ?3 per person. Taxi would cost you maybe ?35 return (including wait time) so only worthwhile if you have companions to share.
If you plan on ascending the falls read up on other threads here regarding suitable gear to wear and safety issues.
Publicos and guaguas can get crowded so if you will be uncomfortable carrying your gear on your lap (and especially if you are a big guy) simply pay to use both "places" (in reality one single seat!) up front next to the driver. It will double your cost in fares but still work out a lot cheaper than a private taxi.
Other disadvantage is that you will have to walk from the main road to the entrance to the falls (about half mile) but if that is a problem for you then you won't get far up the falls.
* I have estimated prices in limey money for your convenience! :cheeky: