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10 Peons are Worth 20x What 1 Is.

The Real Cost of Losing Peons, by Zileas, with data from Frobitz (and lots of other calculations and insights from him as well).

When you lose multiple probes to an attack, the cost of their lost to you increases a great deal with each progressive loss.  Although this doesn't make a lot of sense right away, you need to remember that a dead peon doesn't mine minerals, and if you don't lose peons, they keep mining.   In other words, when you lose a peon, you lose 50 minerals (its cost) + all the minerals you COUDLVE mined in the time it takes to build a replacement.

Frobitz found that a probe mines aproximately 12 minerals in the time it takes to build a replacement probe, on average, on ladder maps.  Keep in mind this number may be marginally lower for slower accelerating SCVs and drones.  So what does this mean?

Well, consider this:  Player A loses 9 probes to a reaver drop (which is a reasonable number.  You can almost always get 2 scarabs into the probes,  particularly if you suicide the reaver).  Heres what Player A REALLY lost, assuming that they IMMEDIATELY build their probes back up no matter what, possibly ignoring their defenses (which they generally cannot do):
50n + [12 * (n + (n-1) + ... +1)], which when simplified ammounts to:
50n + 6n(n+1)
Don't worry about the math.  I assure you its correct.  n stands for the number of probes lost, and the first term is the cost of replacement, and the second term is the mining you lost out on.  Obviously in the  very very long term (mined out minerals), the second term becomes irrelevant, but since we ALL know 1000 minerals now is FAR better than 1000 minerals 25 minutes from now, the second term IS very important,  especially since you usually only kill probes on somewhat fresh mineral batches.

Just as a reference, I thought I'd just calculate a few sample loss numbers. Losing one probe is 62 minerals; 62 minerals per probe
Losing five probes is 480 minerals;95 minerals per probe
Losing 9 probes  is 990 minerals; 110 minerals per probe
Losing 10 probes is 1160 minerals; 116 minerals per probe

Implications of this Information

With this sort of info, you can often make decisions as to if reaver dropping, or mutasacrificing to kill drones/probes/scvs is really cost effecient.  As you can see, if you are getting a substantial ammount, it definately is.  The most typical way to decimate this number of probes is a reaver drop, or a psi raid.  Lets compare costs.

A good reaver drop, with a reaver and a dragoon (standard island payload) costs you 625/150, assuming you lose all units, including the shuttle (which usually doesnt happen).  assuming you kill at least 8 probes, you are at least breaking even, and since we all know they rarely can produce ALL probes, and neglect defense, this will probabaly hurt them a great deal more.  Also you usually get more than 8 probes with a good reaver drop -- you often get some combat troops too, or even more probes.  A dual reaver drop  costs a bit more -- 750/200.  If you take out 9 probes, you are coming out ahead here.  Finally, consider a single templar with dual storm mana psi raid.  Assuming i get off a good set of two psi storms (which I almost always do) at different parts of their peon line, I can safetly say I'll wrack up about 10 kills.  This is a replacement cost to my foes of 1160 minerals, and 250/150 to me assuming my shuttle and templar die (which they often do not).  Psi raids are INCREDIBLY cost effective later on when you have templars.

Frobitz reminded me of the more intangible things about this.  First of all, your opponent is on the defensive after this sort of thing, which allows you to not only continue dropping them with some avail, but to expand or climb tech.  Also, reaver dropping is time intensive so you suffer somewhat of a concentration cost, although your opponent suffers a good deal of this as well.  Finally, this is sortof psych warfare, which I will write a page on soon.  By running in and decimating their probes, you demoralize your opponent, and possibly panic them, especially if they  fail to kill a single one of your units.  I drove beefcakecartman nuts the  other day, killing his ability to effectively retaliate, by randomly sending  hallucinated shuttles across the map and occasionaly mixing in real shuttles  and dropping his expansions.  Psych-Warfare rocks in Starcraft.  And once  again, a super defensive opponent is a dead opponent.