While this is being presented as part of the robots as spaceships series, this is a tool I've been using for a long time when building spaceships. It simplifies table look-ups, and gives us a lot of power and flexibility we can use to build spaceships that aren't exactly on the SM benchmarks.

## Relative Cost

Every spaceships system has a relative cost. This number represents how expensive a system is for a given size. The relative cost of a system is equal to the cost of a SM+10 version of that system divided by 1 Million. We can also use an SM+4 system cost divided by a thousand, or a SM+6 system cost divided by $10,000. It doesn't matter which method we use, as they all give the same number. A habitat has a relative cost of 10, a fusion reactor has a relative cost of 100, a weapons system has a relative cost of 60, and a wheeled drive train only has a relative cost of 1.When we build a robot, we don't look up and extrapolate the price of every system. Instead we look at the relative cost of each system, and track the total of all those. This is the spaceship's relative cost.

The relative cost for spaceships can vary widely, and sometimes reach 1000, but ground robots (after we take off the weapons, which are expensive and robots as spaceships doesn't use) tend to come out between 50 and 250, as the most expensive spaceship components tend to be reactors, engines, and star-drives.

After we have the relative cost for a spaceship, we multiply that relative cost by the base cost for the space-ship's hull size.

## Base Cost

Each Spaceship SM has a base cost: a number we multiply the design's relative cost by to get the actual cost.Weight | SM | Base Cost | Weight | SM | Base Cost | |
---|---|---|---|---|---|---|

3 oz | -6 | $.01 | 100 tons | 6 | $10k | |

10 oz | -5 | $.03 | 300 tons | 7 | $30k | |

2 lbs | -4 | $.10 | 1000 tons | 8 | $100k | |

6 lbs | -3 | $.30 | 3k tons | 9 | $300k | |

20 lbs | -2 | $1 | 10k tons | 10 | $1M | |

60 lbs | -1 | $3 | 30k tons | 11 | $3M | |

200 lbs | 0 | $10 | 100k tons | 12 | $10M | |

600 lbs | 1 | $30 | 300k tons | 13 | $30M | |

1 ton | 2 | $100 | 1M tons | 14 | $100M | |

3 tons | 3 | $300 | 3M tons | 15 | $300M | |

10 tons | 4 | $1,000 | 10M tons | 16 | $1B | |

30 tons | 5 | $3,000 | 30M tons | 17 | $3B |

If we want to just use weight, the base cost is equal to $100 per ton, or $1 per 20 lbs. This helps us get the cost for spaceships that don't weigh a multiple of 10 or 30. We'll need to extrapolate the other statistics for such a ship, but we've been working on that.

We get the base cost from the chart above, multiply it by the relative cost of our design, and we have the cost of our spaceship, robot, or whatever we were building. And if we decide it needs to be a bit bigger, we still have the relative cost, and are one simple multiplication away from making a bigger version.

## Exactness

These numbers are exact! Pricing spaceships using this method gives the exact same numbers as looking up the values on the system cost tables. This is because spaceships is very consistent about spacing its sizes and costs, with the possible exception of weapons. We are free to use variant equations rather than dozens of table. Its a nice feature of the spaceships system.## Exceptions

There are two exceptions to this otherwise consistent pricing scheme of spaceships. Both are found in Spaceships 7, and I'm not sure why they were priced like this.**Gasbags:**For whatever reason, gasbags scale at a different rate than everything else. Perhaps there are engineering reasons this change was made. I'm not entirely sure at this point.**Exophase Field:**they may have had their reasons for gasbags, but why this completely fictional technology is cheaper for larger ships is beyond me. Exophase fields are not likely to come up in the first place, and one of the only systems that hit a Billions Gurps $ at SM+10.

## Take Away

So we calculate the relative value of a system instead of extrapolating it for every size of spaceship. The relative value is the cost of the system at SM+10 divided by one million. Once we have the relative cost of the entire spaceship, we multiply it by the base cost for that SM to get the chassis's price. Its that simple!We'll use this system a lot when building these robots, but this is an old trick I developed for anytime I design spaceships. I hope you find it useful. Happy engineering!

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