Cost to live for three months
I say about $1000USD per month will do you well on the north coast in the Sosua/Cabarete area. This totally depends on what you plan on doing for extra-curricular. $500US will get you a very nice place to stay, with backup power, security, pool, possibly on the beach, possibly a/c...A/C is not a must in the winter, but depends on you.
You can definitely make due with less...just like anywhere in the world, you have such a wide range of living conditions. It is a very broad question, so I have to throw all of these disclaimers in here!!
Being from Canada, I am guessing you want to come down, enjoy the sun, get away from the snow, hang out under some palm trees, learn spanish...close? $1000USD will leave you pretty comfortable in this area for this type of vacation. Hopefully someone from the capital can help with the cost of living down there (the dark side
), but I am pretty sure cost of living in SD is higher than up here.
marc