Are you serious? This is utterly juvenile take on events.
1. Pakistan has nothing to do with Iran and its proxy wars in the ME.
2. Declaring WAR on USA is absolutely stupid. Not long ago, Iranians were boasting about shooting down an American UAV and attacking a Saudi oil refinery; some were boasting about US not having in it to counter Iranian moves. Well, Trump administration is calling the shots now.
3. WAR does not break US economy, rather help it. Military Industrial Complex is an important component of American economy.
4. Assassination of top Iranian general wasn't an impulsive act. This show that US is absolutely prepared and willing to safeguard its interests across the board. In fact, Mike Pompeo have conveyed as much to Pakistani COAS as well.
Pakistan must stay away from this war no matter what. WE have too much on our plate already. Time to seal Iranian border.
Point 1 - Modern conflicts suck every power within vicinity of the conflict zone in. If we should not be concerned then remember the Afghan war that the US has been fighting, we became a front-line state. Don't fool yourself that the US and Iran will not be having, conflicting, demands from Pakistan and in one way or the other ask Pakistan to pick a side. Depending on how Iran responds, Pakistan is a very easy place to conduct asymmetrical warfare, considering there are numerous American citizens, diplomats etc. Assassinations of these people in Pakistan would be a responsibility on Pakistan to tackle and that would be a key US demand which failing would make the US intervene and violate Pakistan's sovereignty. So, stop being "juvenile" in your thoughts on this matter.
Point 2 - The Iranians did not declare war on the US, the US declared war on Iran, its the other way around. The act is profound that everyone around the world knows that the US committed a major act of war and that Iran is now justified to respond and retaliate. Also, Iranians never boasted about Saudi oil refinery attacks and rather claimed to be non-complicit in that event, go check your history.
Point 3 - Wars always break economies. The US is now weaker than it ever was before 9/11 and subsequent wars it carried out as a result in the context of War against Terrorism. Material losses and manpower losses contribute to economic loss. A show of force itself is a significant expenditure such as the moving of US Naval battle group. If the US uses F-35s and if even one of them gets lost, that is a significant economic loss. If the US fires a battery of cruise missiles against targets of less value, that is an economic loss. Remember, the people paying for these wars are the tax payers, the more tax they pay the less purchasing power they have and so the less they spend in the economy. The only time wars are profitable to the US when it is an arms supplier to two warring factions.
Point 4 - Yes, this was an act by the US where they clearly weighed their options and are working towards convincing how it was a legitimate affair. Keep in mind that Iran has also declared the US armed forces in the regions as a terrorist entity. So, by US practice, the Iranians would seek similar outcome of claiming a US general as well. The US is completely capable (and has) to carry out such operations. Likewise, Iran will now be able to carry out the desired outcome with the capability it has in its hand (which people know and dont know). Iran does not need to be as overt as the US in achieving its outcome. For retaliation, they can kill a US general on US soil, at a time and place of their choosing, through third/fourth party actors, with the appropriate lethality, to achieve the desired outcome.