On a professional level -- 3,340,000
On the professional level -- 2,700,000
At a professional level -- 2,770,000
At the professional level -- 2,990,000
EDIT: I agree with Arkalai's differentiation between the use of a and the with this phrase.
It also seems (not just according to Google, but also to my ears) that on is used more when speaking generally and using the indefinite article, while at is used more frequently with the definite article, particularly with sports.
John Smith won the gold medal at this year's Olympic Games, and will go on to compete at the professional level next year.
In order to keep office gifts on a professional level, be sure not to give anything that could be considered distasteful.
I was imprecise. If there is only one professional level, the correct article to use is "the". The indefinite article "a" is correct if there is more than one professional level. As to the preposition, still talking about sports, I think "At the professional level" and "At a professional level" are more common in AE.
The definition of "professional" being used in the expression "on a professional level" is often this one: "adjective: characteristic of or befitting a profession or one engaged in a profession (as in 'Professional conduct')". In this case, there may in fact be multiple levels of behavior, conduct etc. which are considered professional. So here the article "a" is correct.