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You have to balance return on investment. For me spending $100K was a good investment. It was less than 2% cost of business, that equation will differ, so you need to do your own numbers!
Answer to this and your question is relative to the need, requirement and budget of the client you are dealing with.
In addition, almost all designers charge different rate for their work. It might be wise to first think about what is needed and then approach different web designers and get their inputs and 'add/edit' your requirements for the web site based on their suggestions.
I once saw an ad on the side of the road that said "5 Page Web Site Design for US$ 200.00" & I have heard about companies that have paid over 80k for web designing.
So in your case, the best suggestion would be, figure out your needs, visualize future growth, and consult different designers to get idea of how much budget you will need.
Hope this helps.