If you own a business you probably value your time as much as I do. Stated a different way, your time is very valuable. The time spent by your staff getting their work done is valuable too. The value of each employee, (not their direct cost), is your sales volume divided by the number of employees. That can be a really large figure, in some cases, but it's hard to run a business without the employees. If you divide that figure by 2080 you have a rough idea of the value of each employee for each hour they work based on a 40 hour work week.
A customer, (one who knows very well the value of a good computer), once told me that what he spent on computer hardware was cheap when compared with the cost of the people using them. I couldn't have said it better myself.
How often have you called a company and had them ask you to wait because their computers were slow or possibly not even working. It's frustrating, particularly when you are in a hurry, but can you imagine that it could be a problem with your business?
The bottom line is that spending a little more for a better computer will eventually cost you far less due to productivity improvements for the life of the system and the additional costs associated with putting a new system into service.