Well, the second phrase could be serious. The first is irony.

Sorry, I'm afraid you have it wrong. Neither of these phrases are either inherently ironic or humorous. "Thanks for nothing!" is usually said when the person is irritated, upset, or hurt by a response from someone else. Example: "You call that advice? Thanks for nothing! That didn't help me at all!" "Thanks for nothing" can be used for lighter situations too. Example: The man drove around for 20 minutes trying to find a building. He knew it was on Washington Boulevard and that it was between the 300 and 600 block, but that's all he knew. At a stoplight, he asked a pedestrian if he knew where the building was. The pedestrian replied "It's on Washington Boulevard." The driver, frustrated, responded "Thanks for nothing. I already knew that much." In this case, either the pedestrian was mocking the driver, was providing the only information about the building he had, and/or was unaware that he was already walking on Washington Boulevard. "Thank you very much, I'm sure" is not a phrase Native English speakers use. The "I'm sure" at the end makes no sense to me in this context.

Both are sarcastic (and meant to offend)!

1) Thanks for Nothing: (idiomatic, sarcastic) Expression of displeasure towards a person who has not provided what was wanted. 2) Thank you very much, I'm sure! translated to German is 'Danke für die Blumen'. An idiom means Thank you for the compliment