You're asking several different questions with this one posting.
Sustainable Business Models
There are many sustainable business models that can be developed in HR. Does the client have a current HR Department, or are you becoming the key HR person making decisions and following up on requirements? I'd say that this first question depends entirely upon the size and complexity of the needs of the client you're addressing. If this is an SME, less than $100M USD per year in volume, you're probably looking at less than 1000 employees, so any services you offer might be more focused on individual attributes of the specific employee rather than overall HR needs. I'd address specific management needs through a study based on—most likely—a 360-degree evaluative process.
If you're working with a larger employer, you're going to be looking at the various general situations that exist with HR and addressing each of them in order of importance. The order of importance will be fixed by the employer, by the legalities of the situation, or by your own knowledge and experience in the importance of the challenges faced. If the company has an evaluative process built into its assessment phase for employees, a full study of both the evaluative process as well as the key employee base is warranted. The next review and changes I'd probably address are the key elements of motivation, which drive the revenues and profitability. What is the overall general tone of the work environment? Measure this objectively for a full management report.
Developing Competency and Quantifying Value (DCQV)
Next, I'd focus a study on DCQV, or Developing Competency and Quantifying Value. DCQV is a springboard against which you can measure the workplace output and its value from the Chairman of the Board of Directors to the newest hourly paid worker. Establishing the guidelines for DCQV can take time because measurement and comparison to expectations are sometimes tedious, but once objective goals (no more than 5) have been identified for each individual, you have a baseline DCQV Evaluation which measures every employee across the enterprise. I generally price DCQV Studies in the $50 per employee category, which means that 5000 employees evaluated in this manner would run a company $250K or so and would take approximately 7 weeks.
It's very difficult to offer group pricing, SLAs, or defined cost programs without knowing what the specific needs of the client might be, and I'd be at a loss to give you per group pricing without knowing or understanding the scope of your question more completely.
Specialty/Process/Skills That Differentiate the Purveyor
Immediate aspects of this that come to mind are these—if the purveyor has developed specific measurement tools, like DCQV, above, there can be no real comparison with other purveyors—there is no measurement against something which doesn't exist anywhere else.
One aspect that I always try to cover with clients—many ideas, concepts, and thoughts are not entirely new, but they may be new to that industry, or the application of the ideas, concepts, and thoughts may be new to that industry. This generally makes you a unique purveyor in that you are looking at the application of procedures in a different way or a different manner.
I hope that's helpful.
Regards