As it stands now each part has an assigned cost in the VAB/SPH and the way I think it will go is that the cost of the rocket will be the sum of all the part costs
Love your ideas and the debs have talked about like doing stuff later on where to unlock heavy landing legs or some other part not only must you tea search them you must complete certain missions like landing a probe or a fly by
this is just iteration 1 of the tech tree and I subject to change. it could even change in the next 5 minutes. an argument they also provided was that they are kerbals and aren't the most safety focused as jebidiah has shown us