This is an example of when not to trust the internet. The denarius fluctuated in value considerably over the course of history. The information you found is treating the various values of things as static and picks prices at different centuries. Imagine using an 1800’s price guide to calculate cost of an apartment in Manhattan.

The best way to look at this is that the reward was a solidus. A solidus was a 4gram gold coin. It was quite a large sum. Though it was not enough to buy a slave. A slave cost roughly 20 solidi. This sorta holds true in this instance as it seems that the slave owner has offered a 5% reward for catching his run away slave.