I must admit I have no idea about accommodation costs, I really wouldn't like the pressure hanging over me that I need to reach some pre-booked place that night. I know you could also take a chance and book last minute, but no, that also seems too much for me. I love to just have my accommodation and food with me and no pressure about where I'll be stopping. It also serves to keep the costs down low.
That said, for the last 10 years all our family holidays have been in the campervan and 5 or 6 years ago it was fairly easy to be itinerant and just wander where the whim took us (Living in Germany makes that much easier). Since Covid that seems to have become so much harder. Campsites in Slovenia used to always have space for our van, but recent years advance booking has always been needed. I've not noticed a problem though with cycle camping, campsites can usually squeeze me in somewhere.
Lightweight cycle touring needn't be a problem though. I will admit that when I first dipped my toes into bikepacking, my bike was so badly packed, this picture was from the Slovenia trip and whilst I was camping, it was only a bivy bag and tarp.
The picture below was taken in 2022 in the Swiss Jura after my kit had undergone a severe diet and I'd learnt to pack much better. In this picture I'm actually carrying a tent, sleeping bag, stove and food for several days.
Lightweight camping gear needn't cost the earth, Alpkit for example have some really good kit for good money. The tent I use for example is an Alpkit Soloist, a really good value for money one man tent. Personally I feel that lightweight bikepacking style touring is all well and done if, like me, you want to explore off-road and tackle more demanding routes. Then being completely self sufficient is the way to go, however, with mostly on road touring then rack and panniers are perfectly fine.
Aside from sitting down with maps, there's some great routes collated here:
https://bikepacking.com/locations/europe/
