From what I have written down, the cost will be 69 euro (~$75) for the half and 29 euro (~$32) for the 5K. Based on current currency exchange rates as provided by Google.
I don't recall seeing anything about a challenge. In fact, I would highly doubt it, given their reasoning behind no theme for this year's races. (Paraphrasing) "We want to celebrate running a race here, nothing more than running in DLPR."
And they certainly will have no issue filling every registration spot available.