As an inventor, you might be tempted to be willing to pay the services of a patent agent with a percentage of revenues from your invention. However, this does not fit the business model of patent agents at all. And for many reasons, it is unadvisable to seek such an arrangement.
Patent agents are unlikely to believe that inventions are profitable or not. I guess experienced patent agents know what is likely to yield a patent, and some could warn you or tell you to not file. Yet, they probably know that there is a lot of uncertainty based on what examiners will find during search and how well the application will go through prosecution. Some national jurisdiction are known to grant you a patent if the application respects minimum requirements and the applicant argues strong enough, but such patents are of little value, eventually, as they would have to stand in court. Besides, filing and obtaining a patent is a very long process and patent agents have cost to cover months or years before the patent is even granted, let alone profitable.
Besides, a patent could be profitable in two ways. It could be used to discourage or prevent competition. For example, Miele filed a patent application in 1987 for cutlery trays in dish washers. This patent family was subsequently used by the company to prevent competitors to manufacture and sell dish washers with cutlery trays, establishing some technical advantage until the patents lapsed (nowadays, several manufacturer sell dish washers with similar trays). It was and is still used as a marketing argument. This typical case of profit is difficult to estimate and to determine before the patent is granted. Business analysts at Miele knew much better than any attorney what they could expect from this invention, and protection was just a side-cost.
A patent could also be profitable through litigation. This is a domain that patent attorneys know much better, and those who know about it are unlikely to accept a percentage of future revenue to cover filing fees since filing fees are extremely small compared to litigation fees, and there is no way to find out at filing time if the patent will be both granted and effective during a lawsuit. Please note, however, that some attorneys could accept a percentage of what can be gained from a lawsuit or an agreement as a payment of their litigation fees.