I always stay in the Tai Pan hotel just down the road from Soi Cowboy - and thus very close to Asoke BTS and Metro lines.
Price in your range and nice and quiet.
Or failing that the Indra Regent at Pratunam is nice, too.
And don't forget to go to the Sky Bar outside at Lebua State Tower, even if it's only for one drink. Go early (I think it opens at 6) because it gets packed. Stunning views of the city from 60+ floors up. (Dress smart casual, no vests, shorts, etc.!).