Do you know what percentage of quality leads is influenced by marketing?

About50 per cent

I believe  it  varies between products and services and from product to product as well . In addition, you have to consider in which stage of the product life cycle the product is currently at. Let us start with the services. The portion of marketing for existing services in my opinion ranges between  20%-30% max. Reason is that usually customers depend on referrals and not on advertising or promotions . One reason for that is  the service nature itself which can't be physically touched or sensed. For new services usually the portion is much higher and can equalize the marketing cost in products which I will explain later in my answer.  Moving to the products , you have to bear in mind in which stage the product is currently at . In introduction stage the marketing portion can jump up to70% or80% since  this where marketing plays big role in introducing the product to the prospective customers and spreading product information over the market in order to initiate sales leads. Then once product moves to growth stage and mature stage the portion gradually decreases to50% or sometimes less than that . Then in the decline stage the marketing is usually around10% since it all depends on sales follow up  process and  maintaining  current customers. Then in  most cases the product  goes into another life cycle by  adding some improvements  to refresh the sales and regain the lost share . In this stage the marketing portion increases again but not to same level as in the first introduction stage since the advertising is only focused for the new feature/improvement  not the whole product. In this case I assume it won’t go more than40%. I hope all is covered in this answer. 

So as a summary it can range between10% up to80% depending on some factors.

I think that studies have shown that half of Akaar but samples taken vary from region to region and the concept of shopping and culture marketing vary from community to community so I do not think it is possible to determine the percentage of

I Agree with Mr. Jetley on about the 50%.

40% to55% is my opinion

Marketing efforts contribute majority of portion and may be more than  half of the lead.