For 500 look for your local motoconcho thats the best you are going to do and maybe a beat up 115.
a new domoto 125 (chinese suzuki, and if well maintained prob alright) will sell for around 1,000 $. I know some reputable dealers in the jarabacoa and constanza area for second hand ones. Mind you, warranty is something unheard of in the DR (except if you mean anything between 3 days and a week).
not looking for pretty just need it to run for about 6 months. anyone know what price range i need to look in? looks like i am bit cheap here
ps. i'll be in the Puerto Plata area.
If you can't find a bike in Sosua for a price you have named within 2 hours then you have serious communicational skills. Pop into Chera and they'll make you one up in a couple of hours for $400, might even put a light on it for you as well!