Budgeting is really important while you are getting your website designed by experts. While many people, who are new to the field of web management and ecommerce, have this notion that web design is a onetime task, and after that you may not require the designers ever, this is actually quite wrong. You would actually need the web designers though life if you are a layman, or just plan to stay too busy with the other aspects of the business of the ecommerce site.
The various site types and work needed on them
If the site is a news site, you would need someone to input and update the daily news. If the site is a discussion board you would need manpower to moderate and maintain the forum. If the site is a blog, you would need someone to update the blogs and enter new blog entrees. If the site is a portfolio site, you would have to enter new images, and categorize them too. In this way, every site type will demand some data entry and content entry from you from time to time. Now this frequency of content entry would vary from one site to another as per site design and purpose.
And all these entries cannot be handled by a person who may have many other jobs besides site maintenance. Rather you may be a complete amateur in content entry to site. Hence, your web designer actually has a recurrent job on behalf of your site.
The recurrent job of the web designer
Sometimes the job will be to update content, and sometimes to change the site web design, sometimes to change the sites software, and sometimes to add things to the design, add site pages etc. Therefore the job of the designer will never be over, and the site will always demand updates in one or the other way.
Therefore you can understand this quite clearly that you would need a package which would be within your budget easily, and doesnt make you struggle to pay the designer often. As this is going to be an ongoing relationship, hence the package has to be manageable and easy on you. Besides, you would also have to make sure with the web design team, that they take care of the sites on page SEO each time the site is updated. And this must be included in the web design budget too without the red tape of any terms and conditions.