I might have to give this a go. It sounds like fun to learn and experiment with and should work out cheaper than the ready to grow bags.
Discussion
It's definitely fun & interesting.
You can keep it pretty cheap but you'd want to be growing a few bags to make it worth it.
A huge shortcut would be buying colonised grain spawn bags ready to inoculate your growing medium. At this final stage, contamination is less of a risk because you're dealing with less nutrient rich medium.
Cooked grain & sugar water is so easily contaminated.