Well, I'm riscking to not understand me one more time

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What I mean is most of the experts are not really ecperts so - Just do this, you can even offer free trial (while you learn it)!
My best ROI from adwords were 5% on really low budget adds. The client spent/split 40% from the profits with google and were unhappy as hell, but this is actually very good percent on very cheap products, when adding the value of the returning buyers, SEO, expirence with clients.
First consider the niche:
- profit per sale
- search quantity
- competition
Ask them how many of the proffits or per sale they are ready to split with g00g.
Check how many goals are completed from organic search and divide it on 5 for adwords. The problem is adwords and analitycs showing completed goals late and when buyer check for the current date may see 0. So if the goal completion in analitycs from organic search showing 10% tell them, that can expect 2% ROI from adwords.
Campaigns you can't get wrong with:
- cheap cpc using pages in index
- remarketing (if low traffic site still good to show pottential)
- products using business data (dynamic adds) - low conversations, but the buyers getting impressed from the looks
Always add addon links, description, phone, place in the add even it is not always displayed.