One of the things that you will inevitably run into, many times over, is the proverbial brick-wall in your research. You will reach a dead-end that seemingly has no end, you will have exhausted all available resources with no luck.
I ran into this problem with my paternal line which disappeared into the thick Bavarian forest. I knew that I had enough data to likely make some progress in the small Bavarian town of Sailauf that my ancestors emigrated from. I had copies of a birth certificate, military release and other documents that identified a clear path to this town but then my trail went cold. I knew that the trail could likely be picked up in the Catholic church archives of the region, perhaps my relatives still lived in and around this area.
I also knew that I would never be able to complete the research I wanted from my home in Northern Kentucky so I began to look into options for professional genealogy research. I spent some time inquiring about knowledgeable people who would perform quality work and checked references. This is an important step, I do believe that the vast majority of researchers are honest ethical people this is a market that is prime for fraud. If you think about it, you are asking someone to go and find some facts about your ancestors which you may or may not be able to validate. You can be presented with some intriguing data but not have much ability to verify it’s authenticity. So do your homework and know that you are hiring someone with a solid reputation.
When hiring a professional researcher it is important to find someone who is knowledgeable of the specific geographic area you are looking into. Very often research will be done in church and local government archives that may not be generally open to the public. Hiring someone who is familiar with the are local archives, and more importantly those that are responsible for their safe-keeping, is critical. Outsiders will generally not be able to obtain the same access that a well-known research can obtain.
The cost of a paid research is typically a fixed hourly rate plus expenses, it is not uncommon that a researcher will travel to perform their research and they should rightfully be compensated for their expenses. The total of their hourly rate and expenses should be very carefully understood in advance as these costs can accumulate quickly. I personally have used a professional researcher and was pleased with their results, the extended my paternal line back two additional generations and identified several siblings of my previous primary ancestor. This research also cost me more than I was expecting and if I consider the amount on a per-ancestor basis it is a legitimate question to ask if it is worth paying to know a name and not much more. I do intend ot know more at some point but the new data I was provided yielded little through my traditional search methods and so any additional research will likely require additional investment in a professional researcher. That said, the cost I paid was still significantly less than had I tried to travel to this location and perform the research myself.
There is also no guarantee that a researcher will find anything that you deem as particularly valuable and you need to be prepared for this possibility. You are paying the researcher for their effort, not the fruit of their effort. The bottom line for me is that a professional researcher can be a valuable asset to pushing through a dead-end. On an hourly basis their efforts can be viewed as expensive but if you consider how many hours you might have spent trying to obtain the same data they can also be quite a bargain.