[ScottJ]

Depends on the quality of the traffic and why they arrive, if people are arriving looking for images which a lot of my traffic is then it's harder to sell services like translations and subscriptions, also there are a huge amount of things you can do on site to improve conversion rates so there are no defining numbers imo it's very much trial and error.

You could target a very specific keyword that is easy to rank for and get 100 visitors and 5 sales. 1:20

You can target less specific keywords that are higher in traffic volume that is less relevant and harder to rank for and get 10,000 visitors and 25 sales. 1:400