Great. I`ve seen some experts asking for % of sales increase

Why doesn`t accountant/security guard/delivery man or postman doesn`t ask for chunk of profits - they all help to increase my revenue

Because none of them are stepping in and directly bringing customers to your site.
You are not the first to raise this, and the reason it seems odd is down to mindset.
you have to compare it to what it is comparable to :-
telesales. - they find you clients and rag them in or actually close deals. WHY PAY THEM and not the accountsnt etc?
Off site canvasser - they find you clients and drag them in or actually close deals. WHY PAY THEM and not the accountsnt etc?
Leaflet designer, printer, distributer - they find you clients and drag them in or actually close deals. WHY PAY THEM and not the accountsnt etc?
yellow pages - they find you clients and drag them in or actually close deals. WHY PAY THEM and not the accountsnt etc?
newspaper - they find you clients and drag them in or actually close deals. WHY PAY THEM and not the accountsnt etc?
We had a free newssheet, when we got out, it was making more from our own adverts that from advertising, to the point where we were happy to GIVE advertising away as long as it had offers, why? Because we were earning £75-£120 per mobile phone we gave away (and to give you a better idea we are talking about early Motorola bricks)
An SEO will say 'give me profit share etc, then will step in and inprove traffic, conversions, and thus profitability.
What amazes me is that people complain about paying for a business tool ?
people can think what they think, but the facts are that themore competitive a niche is, the more it costs to rank and convert in that arena. Traffic alone won't always cut it, you have to have a conversion head on as well.
So if a consultant knows that if he puts in x amount of effort, the client will get xx return, then ongoing there is y amount of input. How do they charge?
WHy is it that people are happy to pay 10% on Ebay, but believe it to be wrong for someone to ask for 10% of sales THEY bring in?
The reality is that the wb is its infancy and business people need to understand the opportunities out there and grasp them with both hands.