Joe, tell your sister to try google ads first. The returns will be very minimal, but it will give you an idea. Plus it helps with search engine listings.
The corporate advertisers have been very tight for the last two years - It is far from a recession proof industry. Although we're hoping things will change for the better as the economy improves.
To answer, 120 visitors a day is a very small site. Encouraging for a new site - but generally speaking, only friends will advertise on it. Very difficult to put a figure on it as there are many other factors such as the website's content. But advertisers tend to start looking at sites with 1000+ unique visitors per day. With LU, I used to think 30,000 unique visitors per month was the magic number, but we surpassed that a long time ago and we're still haggling with advertisers.
With agencies, be cautious - as in life - be wary of deals with anyone asking for money. IMO, dealing direct with people you know (and without middlemen) is best.
The best advice is to work hard at building the website and be patient. Again, it depends on the content, but it can take years before the website starts to earn a reasonable income.