According to the IWS (Internet World Statisics) as of June 2008 approximately 71% of everyone living in the United States are internet users.
Can you afford to NOT have a web site?

"Toledo, Ohio is #10 Instant Messaging City in U.S.; "
BNet - 2003
(13-17 93%, 18-34 80%, 35+ 71%)   These are Internet savvy users.   Are any of these age groups among YOUR customers?

The Value Proposition

The Costs

Development

Development costs are dependant on what you will want from and for your web site.  The more you want your web site to do for your business and the more extensive the "look and feel", the more it will cost to develop.  In general, a static reasonable size web site should not cost more than $300-$500.  Based on features, the cost can go up from there.

Maintenance

Maintenance fess can usually be broken into 2 categories.  

The first being those fees your support will charge to maintain and/or monitor your hosting account.  While you could do this yourself, it would be better to let the support handle this.  These fees should probably be in the $5 to $10 per month. 

The second being those fees that will be charged for actual changes to your web site.  Over the life of your web site it is reasonable to assume that your web site will go through numerous upgrades and changes.  When it is time to request a change, be sure to go through the specifications with your support company, have the costs identified, and if all possible, stick to the specifications without changes, additions.  Doing so will help speed the process and reduce the chance of errors.

Hosting

Hosting costs are those charges incurred from your hosting provider.   These charges will range from the annual (or monthly) cost of hosting the web site, to an annual charge to maintain ownership of your domain name.   But as with the development costs, they should be easily determined and not very substantial.   Somewhere between $100 to $300 per year (refer to the Hosting menu selection).  

Internal Business Changes

This is the area where there is the greatest potential for cost increases.   It will really all depend on the goals you set for your web site, and the capabilities you have built into it.   For example, if you use your web site purely to display lists of your products, with no capability for the viewing customer to order, etc from the web site.   Chances are that there will not be a need to incur increased business expense to handle any change in your customee volume.   However, once you add the capability for a customer to do online ordering and bill payment, the need for improved back-office functionality increases greatly.  

The Revenue Side of the Equation

This is a little more difficult to quantify.   If we assume that all you provide your viewing customers is a means to see what you do, it can be assumed that only your walk-up business will increase.   Estimating what the increase will be could be diffficult to quantify, and, you would need a means to identify what portion of any increase is attributable to your web site.   One possibility would be to have coupons that the customer could print, thus proving they found you via the web site.  

If, however, you do provide an order, and payment, capability via your web site, tracking increases, becomes less difficult.   Because your web site provides you World visability, your potential customer base just grew dramatically.   For the sake of discussion, let us make the following assumptions.  
1. You limit your customer base to the U.S. only
2. You end up with a success rate of .00001%.
According to the latest available statistics from the IWS, that would translate into an increase of approximately 2,200 new customers.  
What that means to you on both the cost and revenue side of the equation only you can answer.