An extreme example first should be helpful to you. Let's supppose you are a small business that is on the second floor of a building without an elevator. Let us next suppose that an applicant with perfectly good qualifications, but in a wheelchair, comes along. Installing an elevator, even supposing you own the building, is going to cost you approximately $10,000. That expense would put you out of business, not allow you to purchase sufficient inventory, or perhaps force you to layoff several workers. In these cases, the law is clear that this would be an undue hardship.
At the other end of the spectrum, you might have a person who is hearing disabled and works on the phone as a customer service representative. He needs a device to amplify what he hears on the phone, and the device costs about $30.00. This would absolutely not be considered an undue hardship.
What is expected of an organization is something less than going out of business, having bare shelves, or laying off workers to pay for an accommodation. However, there are no clear lines, and this is a case by case situation.
Another aspect of ADA that you should bear in mind is that the employee must be able to do the essential functions of the job to trigger the accommodation requirement. An employee who has a weight-lifting restriction and whose job it is to lift 100-pound bags cannot do the essential functions of the job and need not be accommodated at all. Similarly, a ski instructor who has broken his leg cannot do the job he is hired to do, and there is no requirement that he be accommodated.
In many instances, a creative organization can easily accommodate a disabled employee without great expenditure, and there is no good reason to resist doing so in order to retain a valued employee or hire a new one who is well-qualified to do the work.