Sometimes we can forget that we are not negotiating with Posey or with any other player. We are negotiating with their agents. Agents aren't as sentimental as players. They will encourage players to seek more money, and often that is good advice. They are professional negotiators, and GMs need to be the same.
Often, the agent will say they want money that they know they might not get, but they have to try. The team and the agent both need to try to get the deals that will be best for them individually.
The idea of signing anyone "at all costs" is bad business, and sounds more like hyperbole than any kind of legitimate suggestion. You don't have to do that and shouldn't do that. You need to get an idea of what the market is, make an offer that you feel is fair, and prepare for the possible outcomes.