Good question. The problem is it depends what your business model is. Shooters working how we did, invested a lot for a very good long term return. I would say 15% to 20% maybe more.
Affiliates can invest very little to get very little back.
Today the problem is what to invest in. A great kick ass site is expensive, yet needs less spent on promoting it to surfers to get the same return of a an average site that needs a lot spend on promoting it to reach the same level of income. Yes there are exceptions.
The problem with the great site method is the bulk of the money has to be spent up front.