For reference, I built my dual Xeon system for $500 USD. That said, I have been playing around with computer hardware for a long while and have lots of experience building systems, so your mileage might vary.
The other consideration is what kind of models you are running - it really only makes sense (in my mind) to fret about hardware if you are running models which are hitting some kind of realistic limit. For most of my day-to-day work, a laptop is more than enough. Even with that larger model, I developed the model and tested it on my laptop with a subset of the data, and only needed the computation for the final stages with the full data set.
That line for me was when after every optimization I could think of the model was still taking long enough that I could not run it over night and look at the results the next morning.