This is always a tough question to answer when a potential client asks it without providing expectations or goals. How much you spend online solely depends on what you want out of it.
Web services range anywhere from building a basic static website with no marketing efforts to creating aggressive online strategies to help reach your target market. In addition, there are different levels of marketing services depending on what online marketing tools are being used. Also, one key factor is if you are trying to reach a local or global market.
If I had to throw a number out there, I would say for the creation of a basic website and online marketing strategy for a medium sized local business, expect to pay anywhere from $600 – $1500 a month for Inbound Marketing Services and possibly an upfront cost depending on the details of the project.
A good marketing company should be able to tailor a great plan and strategy for your business based on how aggressive you want your presence to be.