Hot stones dilate the blood vessels, increase blood flow, increase lymph. All of that increases the effects from the essential oils.
For a new client, I usually do hot stones alone, to see how they respond to the heat, then the next visit maybe add essential oils. If I really wanted to include essential oils with hot stones, I'd only use a half strength blend the first time (pregnancy/child strength 1.25%) Stones intensify everything, so less is more - less strokes required, less essential oils required, less effort required (though more clean up required).
I wouldn't bother charging extra for a half strength blend, definitely charge extra for stones. If you're using more than a couple of stones, it's so much extra work heating up the stones then cleaning them after, I'd say charge at least for an extra 20min, I charge an extra 30min. Eg if you charge 60 for a 1hr massage, charge 80 or 90 for a 1hr stone massage.