The cheaper version even, which may be what you are talking about, is literally just a piece of plastic or metal called an interlock on the breaker box that prevents the main from being on while the generator inlet is supplying power (backfeeding the line). Then all you need to do is install the inlet on top of that which when I did this a few years ago was around $1,000 USD all in parts and labor.

Electricians like their transfer switches because they are right sized and everything, e.g. they are more foolproof for the end user to operate, but the interlock approach actually gives you more control because every breaker is allowed (a transfer switch will only expose some small subset of all your breakers usually), you just have to manage the power supply yourself.