Is there a way to make a race without a constitution score that is not using the undead or construct type? It seems like since the types have an RP cost, you should be able to break down what each element of that cost is, but I can't find any way to add this to a character without that type. For example, there's a "Half-Construct" subtype, but say I want more of a 2/3 Construct. Is there any way by RAW to do this?
The written rules are for full or half. If neither of those is what you want, you're going to have to design it yourself. In this case, the RAW is literally what's written, full or half.
It shouldn't be too difficult to look at those two and figure out a mid point, benefit and cost wise.