You are already paying for expensive resources: computers, employees, internet services, and more.
Whether you're paying the right amount for each of those items is another matter. A good consultant will give you ideas and solutions for places where you are either spending too much, not spending enough, or are not taking advantage of some revenue-producing opportunities.
And a good consultant can help you prevent making big mistakes with your technology investments.
If you focus on results and the investment in building your business, then you probably can afford a consulting firm.
If you're wanting to know hourly rates before you ask "what value is this firm providing me?" then you might not be ready to benefit from working with a consultant.
Not sure? Ask for a reference client that matches either your industry and/or company size.

