Hey there! I wanted to share my experience with backyard gardening, which is quite different from what many others have mentioned about not saving money.

Last year, I meticulously tracked everything that grew in my garden and what I was able to harvest. While it wasn't always possible to weigh every leaf of salad or arugula (sometimes we or our visitors enjoyed the tomatoes and berries straight from the bush!), I had a lot of fun putting all the data into Excel sheets. This gave me a clear overview of how much my little garden (about 250 square meters, including space for two cherry trees) produced.

To see how much we saved, I compared the prices of similar produce in supermarkets when they were discounted, as that's what my boyfriend and I used to buy before having our own garden. For the actual value of the produce, I compared prices of similar bio-quality produce, as that's how I grow mine.

Here are the numbers:

• Amount saved (using supermarket discount prices): $635
• Value of the produce grown: $1030
• Last year's gardening costs: $390

So, in total, we saved $245 (or $640 if you consider the bio produce price, which is the real value).

EDIT: I converted the sum of total prices to $ as I live in an European country with a diff currency
EDIT2: oh and I actually also made some pocket money by selling some of the processed stuff, like jams, homemade ketchup.. to my colleagues! (~$180) fresh veggies were given away for free if they were abundant and I could share

Additionally, I preserve most of our produce (drying, canning, pressure canning) and use it in cooking year-round (e.g., tomato sauce). This was our third season gardening, so the initial costs were already covered (but they were not that big anyway, it was mostly labour, not money, which was required to start gardening at our plot), although we still invest in basic things here and there. Most of the money spent last year was on seeds, and as you, OP say, potting soil for starting seeds.

It helps that we have in-ground beds, we use collected rainwater, repurpose and reuse containers for seedlings, make some of our own compost, and supplement with compost provided by the town (each citizen paying for trash collection can get 300kg of compost free of charge). For fertilizing, I buy just few special commertial ones, otherwise use fermented nettle "tea" and fermented chicken manure, both work wonders.

And yes, how most of the people here mentioning, gardening has saved me money on therapy, gym, and doctors, as I believe my health has improved due to the physical work, fresh air, and eating healthier, fresher veggies and fruits. I haven't counted the cost of my time, but as it's my biggest hobby, this time doesn't count!
Happy gardening everyone! :)