This straightforward question but doesn’t have a straightforward answer. Traditionally caffeine levels in coffee depend on dilution or coffee to water ratio. For simplicity, let's take a general approach to this.
Decaffeinated coffee: 240ml of decaffeinated coffee will have a couple of mgs, maybe 2mg of caffeine, which is almost negligible, but not zero
Cold Brew: Next we have a cold brew. Cold brew is where the dilution has the biggest role to play. A 240ml of cold brew can have anywhere between 150-250mg of caffeine; again, depending on if it’s a concentrate or diluted with ice or water. Many people say a cold brew coffee has a lot more caffeine than a hot brewed coffee, but a fully diluted cup of cold brew that is ready to drink is going to have more or less similar levels of caffeine to a hot brewed cup of coffee. It is however more likely for your cold brew to have more caffeine than your hot brew purely because a cold brew is a more concentrated form of coffee.
Instant coffee: Let us look at instant coffee. Instant coffee by the virtue of the technology utilised in making instant coffee has a lot lesser amount of caffeine than freshly brewed coffee, hot, iced, or cold brew. Instant coffee, for the same amount of coffee, would have roughly about half the amount of caffeine. So, a 240ml of instant coffee that is not freshly brewed will have 60-65mg of caffeine, again depending on if it’s a Robusta or an arabica blend, whether it has chicory in it or not. If it has chicory, it is going to have a lesser amount of caffeine.
Espresso: Finally, we have a single espresso shot. A single espresso shot is going to have the same amount of caffeine as half the cup of hot brewed coffee, very similar to a cup of instant coffee, about 60mg.
It is important to note that we are talking about 100% Arabica beans. If you use Robusta in any of these coffees, your caffeine levels go up because Robusta has double the amount of caffeine when compared to an Arabica bean. By weight, a Robusta bean would have 3.5 - 4% of caffeine and Arabica would have 1.5-2% of caffeine.