We do marketing partnerships with some of our ongoing accounts. Terms vary from fixed-fee retainers through fixed amount + 10-20% split share to 30-40% fee on new deals.
Depending on the specifics of the business and the marketing services provided, the percentage could be for a specific product/service, or the entire business. We do inbound marketing and partnership outreach which increases the brand awareness, grows the email lists of our clients and allows for scheduling cross-promotion campaigns and webinars that support the entire business - hence the added benefit of shared revenue across all products and services.
Limiting revenue share for a product is more applicable for larger businesses that have an established marketing and advertising strategy and bring a good number of customers this way. Then a marketing strategy could be shaped around a specific product or a solution which justifies partnering only on a specific venture.
It depends on the service that you are performing and the budget of the campaign. In my agency, we typically stick to monthly minimums, so we know we are getting a least a certain amount per month from each client. In general we might do a 20-25% of the monthly advertising budget as our management fee, as long as the fee is at least $1500/month. The only exception is if the company has a small budget, so we may structure our fee on a sliding scale to sore of help them out while they are smaller and growing, then grow the fee over time as the marketing campaigns grow. On the other side, if the client has a large budget, say 100k and higher, we may have a cap on a the amount we are charging per month, somewhere in the $5k to $10k range depending on how much management work is involved in the management engagement. I hope this helps.