Power Stone Price

[Lord Morpheus]

Please if some on cam help, I will be glad.

In the GURPS Magic, p.20, we find a equasion to calculate the cost of a powerstone.

The material cost is calculate like this:
(10xP²)+(40xP) | Where P is the powrstone capacity of the powerstone.

The labor cost is 20xP

Ai meke the calculations for every stone in the table, and just the first one is correct. The other ar wrong, at start with a small diference, as de2 pt Powerstone, the cost in the book is 165, but if we use the formulas we find 160. Until the 6x more expensive in the last on (100), the cost in the book is 675.000, and if we use the formula that results in 106.000.

Just tu make a exemple, lats get de 50 Energy point stone.

Material
(10xP²)+(40xP)= $
(10x50²)+(40x50)= $
(10x2500)+(2000)= $
(25000)+(2000)= $
$=27.000

Labor
20xP=$
20x50=$
$=1000

Material+Labor=
27.000+1000=28.000

Total 28.000 => cost in the book 69.5000!

Do someone knows what happens? Am I doing something wrong?

[Anthony]

Originally Posted by Lord Morpheus

Do someone knows what happens? Am I doing something wrong?

You're missing "Powerstone prices account for this risk; hence, the true materials cost is divided by 53/54 to the power of P."
The actual cost of a 50 point stone is:
materials: (40 * 50 + 10 * 50 * 50) * (54/53)^50 = 68,745
labor: 20 * 50 = 1,000.
Total 69,745, which inexplicably got rounded to 69,500.

Note that the box on Magic 20 assumes that quirks don't influence the value of powerstones, which is unrealistic, and ignores some more complex math for the actual number of castings required to produce a powerstone.

[Lord Morpheus]

Originally Posted by Anthony

You're missing "Powerstone prices account for this risk; hence, the true materials cost is divided by 53/54 to the power of P."
The actual cost of a 50 point stone is:
materials: (40 * 50 + 10 * 50 * 50) * (54/53)^50 = 68,745
labor: 20 * 50 = 1,000.
Total 69,745, which inexplicably got rounded to 69,500.

Note that the box on Magic 20 assumes that quirks don't influence the value of powerstones, which is unrealistic, and ignores some more complex math for the actual number of castings required to produce a powerstone.

Thanks I was hopping someone will get this in the discussion. I really don't understand this part of the math. (54/53)^50
What is the calculation here. Can you make this step by step?
Thanks again (my math abilities are a disgrace)

[Anthony]

Originally Posted by Lord Morpheus

Thanks I was hopping someone will get this in the discussion. I really don't understand this part of the math. (54/53)^50
What is the calculation here. Can you make this step by step?
Thanks again (my math abilities are a disgrace)

There is a 4/216 (1/54) chance per try to blow up the power stone, meaning there's a 53/54 chance per try to survive the attempt; 50 tries means (53/54)^50 chance of survival (because a failed roll doesn't add a mana point, it should actually be 206/210 ^ 50). Thus, you need to adjust the cost for the chance of blowing up your stone.

[Flyndaran]

I don't like Magic's assumption that every one of the inexplicably common enchanters has over 20 skill with whatever they're enchanting.

[Anthony]

Originally Posted by Flyndaran

I don't like Magic's assumption that every one of the inexplicably common enchanters has over 20 skill with whatever they're enchanting.

It's irrelevant to powerstones; you only need a skill 16 master.

[Flyndaran]

Originally Posted by Anthony

It's irrelevant to powerstones; you only need a skill 16 master.

But the price still includes basic craftsman's prices for essentially medical doctor level skill. You aren't going to hire a physician for a craft fair worker's salary.

[Anthony]

Originally Posted by Flyndaran

But the price still includes basic craftsman's prices for essentially medical doctor level skill. You aren't going to hire a physician for a craft fair worker's salary.

It still doesn't matter very much for powerstone enchantment, because powerstone costs are dominated by material costs.

[Flyndaran]

Originally Posted by Anthony

It still doesn't matter very much for powerstone enchantment, because powerstone costs are dominated by material costs.

Scarcity of production is quite important to local price, unless the Gurps assumed magic setting is simply crawling with mages.

[Lord Morpheus]

I fill so stupid, I stile don't understand the math, I thin wiil be more easy creat a new enchantment system that understand this one...