I think that's the fun part.
the longer the building takes, the more money I would be willing to spend. I still view modeling as a time/ dollar equation.
to answer the question (which seems important), I would spend up to $2.00 per hour for modeling entertainment. so if a model took 100 hours to build, I would consider spending $200 for the kit, paint, sundries (blades, sandpaper, thinner), photo etch & resin accessories.
most of my hobbying costs me less than a dollar per hour. i'm slow, like to research, and enjoy the process.