If we don't reward them, then in the future people will steal or sell these treasures, or attempt to extract them without the proper tools or knowledge. They should not get to keep what they find that in part of a sovereign nation's legacy. However, a justifiable reward is appropriate.
Of course they should be rewarded for finding an ancient relic. It isn't like these things just pop up in plain sight everyday. What the reward should be, I don't even know. But they should certainly be honored and held in high regard for their ancient find. Things like these are what allow us to know more about the way the world used to be.
Archeologists shouldn't be doing their jobs if they are in it for some type of reward. It is great that they found a sundial that is that old, and they have contributed a lot to society finding it. That should be a reward in and of itself rather than something monetary.
Amateur archaeologists who discovered an ancient sundial have already gotten the reward of being the first people since as far back as 6000 BCE to see this artifact. As amateurs there is no expectation of payment and presumably they chose archaeology as a hobby because of the excitement of finding objects like the sundial.