I think the Addon left off at least half the cost of new construction, let alone an addition, or, maybe that was what they paid in the 70s. If you can get materials and labor for $50, more power to you. Here, costs start at just under $100 and go up from there depending on quality, complexity, fit and finish, plumbing, electrical etc.
Additions can cost from a little more than new construction to twice the cost due to constraints of fitting and working with the existing structure and the cost of demolition. An addition can be a perfect solution. I think the argument for buying new, rather than adding on comes from the fear of over-building. If your property becomes the highest price or value in a given market, that creates problems for recovering your investment. As long as you keep an eye on your total investment costs, and your relative position in the real estate market, that you can make the right decision.
I have done a number of additions at different homes, from kitchens and bedrooms to master suites. They have all worked out in terms of functionality and cost-recovery; and we got to enjoy the use of those additions while we lived there.