What is the construction cost of a Construct Companion?

[Reksew_Trebla]

Construct Rider

Building and Modifying Constructs lists an upgrade - increasing HD - that for a kind of construct not already listing a gp formula cost for increasing HD, that to find the cost for each HD, you divide the construct’s construction cost by its existing Hit Dice.

It may not be RAI, but we aren’t discussing RAI, but RAW, and as far as I can see, a Construct Companion’s construction cost is 0, which when divided by any number (other than 0), still results in 0, which would mean that by RAW, for every 2 HD your Construct Companion gets, you could raise it by an additional 1 HD for free.

Now that obviously isn’t RAI, but nowhere in the Construct Rider’s class abilities does it say you can’t upgrade the construct, and this is a valid upgrade for said construct. So now we are back to the root problem. What is the construction cost of a Construct Companion?

[LordKailas]

The construct gained via construct rider is a class feature and is therefore not really subject to the normal construct rules. The very first line of modifying constructs states that standard constructs can be modified. The construct rider mount is specifically a one of a kind masterpiece.

[Reksew_Trebla]

LordKailas wrote:

The construct gained via construct rider is a class feature and is therefore not really subject to the normal construct rules. The very first line of modifying constructs states that standard constructs can be modified. The construct rider mount is specifically a one of a kind masterpiece.

This seems right, but the part about it being one of a kind seems wrong. Literally anyone can take this class, and the construct is destroyed at 0 hp, which means it has to be replaceable.

[Nyerkh]

Not every rider builds their mount the same way. Some will be metal, some will be wood, some will be stone or meat or whatever else. And they are linked to their makers. That's a very one of a kind, "there are others but this one is mine", approach.

The archetype doesn't say what happens when it's destroyed and how to replace it, true.
Considering it mostly works like an Animal Companion, I'd go with something similar : 24 hours of work. In a lab/workshop rather than a natural environment, but still. No monetary cost, as it's implied you have all you need with you anyway, for constant tinkering, enhancement and repairs.
Alternatively, steal the Promethean Alchemist's way to rebuild his contruct companion, at leats the gold cost part of it.

As for upgrading it ... Considering the Black Blade precedent, or said Promethean's Homunculus, that's gotta be a no.
Class features bend the rules and this is very much not a normal construct, on a lot of levels.