That is pricey for a scooter rental but it is true that you could pay for a cheapie Chinese bike in a month. I bought my 2010 125cc Chinese bike, brand new, for 29,000 pesos in Gaspar Hernandez. ..........If you want a Honda, or Yamaha, expect to pay double.
Having recently (03/12) shopped for a moto on North Coast to give to our maid, prices here are higher. Cheapest are the ones with the donut wheels, but with poor roads those are dubious choice unless very limited usage. With the large wheels, cost for a 100 cc automatic started at about RD$34,000; cheaper ones were lower power.
I finally opted for a Chinese "name brand" Loncin automatic with nicer features for RD$39,000. Add another RD$500 for metricula, RD$500 for transfer and RD$300 for insurance, so out the door for RD$40,300. (OBTW, these fees were posted on an well worn sign for all to see, not just Gringa costs.) I had a trusted Dominican friend helping with all of this as well.
Be aware, I was offered a somewhat lower price for an adequate (but not as nice and a demo) bike, but then they were going to add on 16% Itibis! And was quoted more for fees. Unlike the USA, this tax is USUALLY already included in the quoted prices, but ask about that and any fees so you are comparing "out the door" prices.
Also learned that motos only pay once for tags, not every year like autos. And yes, that insurance is only liability and minimal at that. But titled to our maid, so adequate for her. If stolen, it's also her problem. But was told one cannot really insure against theft with so little recovery chance by insurance companies on these cheap bikes.