The cost is 50 pesos for dominicans and 100 pesos (a little less than 6 dollars US) for foreigners, Round trip. What they do is this: you get 2 tickets, going up and coming down legs of the trip. Each ticket has 50 pesos printed on it, so if you are a foreigner, you pay 100 pesos (50 for each leg of the trip). However, if you are a dominican you simply hand over 50 pesos and you will get the same 2 tickets for the half price.
On weekends you would have to wait a little but on weekdays I would imagine you will get on right away. The ride takes about 20 minutes each way through the most scenic climb of your life. The whole city of puerto plata is visible from there on a good day and the ocean is seen in its most natural aqua-green shades. Ones on top, you will feel that its quite cool and windy but comfortable. The clouds cover the top every once in a while but you get to see the whole magnificient view of the countryside and the city at the same time.
Its a fun trip to take so make sure you eat something first so you won't get hungry up there. Food is sold in the form of simple ham & cheese sandwiches and cold sodas and bottled water.
Have fun.