This is a bit of a 'how long is a piece of string' question.
If you are launching ads in a very competitive sector and you are not willing to pay to have your ads up in the first page. Then the answer will be a small increase.
If you are willing to have your add placed highly on the front page, then the answer is you will probably see a big increase.
This assumes you word the ad in a way that gets people to click through to your site.
It is no good appearing first in the paid ads if your ad text is such that it does not attract visitors.
Good ad text will attract more visitors for an ad that is placed down the page, than for one with bad text that is higher up.
It's as long as a piece of string.
One thing to bear in mind is, if you are in a competitive sector the costs can mount pretty swiftly and the goal is to gain conversions to sales and not just visitors to your site.
It's all a matter of trail an error. Best thing to do is trail an ad, watch it's performance, tweek if necessary.... repeat, repeat, repeat until you are happy with the level of ad spend compared to income gained.