I have been a 3D Artist for a very long time now freelancing for all sorts of clients and have done countless modelling jobs for small and big companies. The problem is not the client but you in this case. Never start work without an agreement in place. Meaning, give the client an estimate on time/cost. If he is OK with the time/cost factor you need to ask for $25-50% upfront payment before you start work. Give the client regular updates on progress with screenshots, videos etc. This is how I do business and have encountered very little to no issues with my clients. Never do work on good faith but proof that you are willing to work with them on pricing. Lastly, don’t send the client any files unless they have actually paid 100%.
As for estimating costs for 3d printing their models. This can be done by simply giving a cost based on bounding box/volume. If they have an idea of the size (width, height, length) and the material they prefer you can calculate based on that but state that this is an estimate and the final price can be higher/lower.
In other words, 3d modeling and 3d printing are two separate jobs and should be handled as such.
