As you can see, buying potions in Diagon Alley is usually similar in price to buying all the ingredients, putting in the Master Notes, and then paying to immediately finish brewing. The two exceptions are Potent Exstimulo Potions and Tonic for Trace Detection, which we’ll get to in a moment.

Of course, it’s rare that you’ll have zero ingredients and zero time, so the real question is at what point does buying missing ingredients and/or just paying to hurry up the potion become the better option?

For most potions (all but Potent Exstimulo Potions), that point comes once you have one or two ingredients (or none, in the case of basic Exstimulo potions which are always more expensive in Diagon Alley) or once you have some time to spare, usually about an hour.

In other words, unless you are doing a ton of back to back fortress battles and burning through potions and ingredients faster than you can brew and collect, purchasing individual potions in Diagon Alley will be the more expensive option.

When Diagon Alley is Cheaper

But what about Potent Exstimulo Potions and Tonic for Trace Detection, you ask? These potions are the only ones that end up cheaper in Diagon Alley in many cases, due to high ingredient costs and long brewing times.

First, Tonic for Trace Detection is pretty straight forward. If you're out of Erumpent Horn, just buy the potion from Diagon Alley. It's the exact same price as the horns, or if you buy the bundle, it's even cheaper per Tonic than buying Horns.

With Potent Exstimulo Potions, it's a little bit more complicated but not buy much. The single biggest cost is the Unicorn Hair, followed by time. If you’re lucky enough to have plenty of Unicorn Hair (30 gold) to spare, this potion is just like the others: roughly the same in Diagon Alley, and quickly becomes cheaper to brew yourself the more time or ingredients you have. For example, it is still cheaper to brew if:

You have all the time in the world (41 gold) but no ingredients, or

You have no time at all, but you can buy the missing ingredients for less than 32 gold

Of course, players will usually find themselves somewhere in between the two extremes, but this should hopefully at least serve as a baseline for quickly estimating which is cheaper on the fly—without needing to pull out a calculator and do some algebra.