If you take a closer look at bolted spinach plants you’ll find the seeds grouped in small clusters:
The seeds can easily be rubbed off the stem although some will stay together and you’ll need to separate those. From a single spinach plant I saved enough seeds to fill at least two normal seeds bags, maybe even three:
I pay about $4 for a single bag of spinach seeds. I now got seeds worth 5 * 2.5 * 4 = $50! (I had 5 dried plants hanging beneath my roof). Amazing. If they will all grow, that is.