I get asked this kind of question a lot. And the information rolling around online completely fails to give a clear answer to this question for a business owner or startup.
After years of experience designing sites for small business owners, and after observing their results over and over, I finally devised a formula which can be applied in any situation, to any level of web development.
First, understand that marketing and web design go hand in hand. You cannot pay for one and neglect the other, or your investment will be useless - for it to work, you must invest in both.
This is the formula I devised:
How much do you expect to earn (in profit) from your website in the next year?
Divide that number by four.
You should be willing to spend that much on website design and basic search engine optimization (which should mostly be done as the site is built).
You will have to spend another fourth of the amount on marketing to get the site launched and stabilized - later you can spend a smaller percentage, but to get it started you'll need to back it with a sound marketing campaign.
Any good web designer and marketing consultant team should be willing to give you reasonable assurances of meeting the goal of doubling your investment within 1 year of the launch date (when the marketing campaign begins). Most of the time you can do much better, but if your product is sound, you can almost always meet this goal.
This means, if you spend $150 on a website and marketing, you really have no reason to complain if you make only $300 from it the first year. Your site will still work, and it will still grow from a small investment, it will just be MUCH slower.
Long term, your ongoing costs for maintenance and marketing could be closer to 5% of your profit margin each year, often far less. The higher cost applies to getting started.
The purpose of this formula is not only to give you an idea of what you should expect to pay, but also to help you understand the minimum you can expect to get in return. Many web designers and marketers are reluctant to give any kind of assurances of returns. We all know that it is slow, and many small business owners give up before the website starts to pay if they did not have sufficient funds to compete right away.
Even if you market on the cheap - most of my clients do their own marketing - if the strategies you use are sound, this formula will still work! Most of my clients in fact do much better than this, but it is due to their dedication and drive.
A website and marketing are well worth paying for. They are the backbone of online business, and they can return your investment many times over.