...website a few days ago, and within 3 days users go up from 90 per day to 400 now, and seems OK to go to 1000 soon. From the statistics, they view about 10 pages each time they visit.
That's an impressive start. Where is your traffic coming from - how are they finding you? The source of your traffic will have an impact on your success with ads on your site.
I'd like to put a small google ad on the site. From your experience, what's the reasonable level of return for a small website like that?
The ads will be based on the content of your site, assuming there are people advertising things related to the content of your site. If those visitors you are receiving are interested in the subject of the ads, you will earn some income. How much income depends on a number of factors and probably a lot depends on the answer to the first question above.