They take about 9 months to start fruiting, and will produce about 95% of the weight of the log in mushrooms (hardwood is HEAVY) over 3-4 years. At $4/lbs you'll get several hundred dollars out of each log! On the expense side there is just initial spores (negligible relative to output), water, time, and the logs themselves, which can be expensive if you don't have your own source. Mushrooms are a great cash crop for homesteaders and their ilk!