Great Question.
I recently bought a Wiim Pro for $100. It’s intended for my basement HT system, which had been Physical Media only. It sounds great. I moved it to the big two channel rig and an experiment running it into same Bryston DAC as my CA CXN 100 streamer . The difference between the two transports is quite small sonically.
I am tempted to say that it must be relatively cheap to build a good streamer. These are just networking computers in audio casework. Networking Computers are incredibly ubiquitous. There must be billions of them in use on the planet. Presumably the essential parts must be driven to the floor price wise. Now we all know that computers are noisy. However eliminating some of the noise making functions of a PC, such as email and texting and all video functions, separation of some of the other functions from each other, perhaps some separate power supplies, and you end up with something that sounds a lot better than a bog standard PC.
DACs on the other hand might be more expensive. Audiophile grade DACs are much less ubiquitous than networking computers. The best chips are not cheap compared to something like Dell Computers might use in a laptop. The parts needed to synchronize the timing, so critical in music reproduction (compared to what is needed for a home printer, for example), have a cost. Power supplies also have a cost for quality. And then there is the issue of inputs, particularly if a DAC is accommodating some something like IS2