The success of an affiliate marketing strategy depends on how many referrals you’re able to send to merchant sites and how well these referrals convert (hence the bolding of these factors above). The more relevant and appealing the offers you highlight on your site, the higher both your click and conversion rates will likely be. If you’re running a travel blog, you probably don’t want to be featuring affiliate offers for baby products; replacing them with affiliate links to cruise packages would probably result in a higher referral rate.
But to go back and answer your question as far as typical ROI, I would just say roughly if a business implements a referral program, you've got to keep in mind that typically you're not paying these affiliates any upfront fee. You just paying a percentage commission for a referred transaction, whether it's a dollar amount, whether it's a percentage of their order total.
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