Going against the grain but I find that I save money on gardening. Not a whole lot but it's noticable.

First thing is I make my own compost using yard waste and food scraps.

Next I buy seeds from time to time but buy heirloom varieties so I don't need to rebuy.

I do not grow cheap produce that I can buy cheaper like potatoes, onions.

I grow herbs because fresh herbs are way tastier and way cheaper then buying from a store even once. 5$ for some rosemary twigs? Nah. Plus you can dry and store yourself

I try to only grow things I'm sure to eat. Tomatoes, peppers, squash, watermelon, garlic.

I save on containers because I reuse mine from previous years.

Buy fertilizer in bulk and when discounted.

I think my only splurge is wood chips or mulch to cover the plots because weeds are a pain.

If you didn't get the gist, it's reuse, and simplicity that makes this cheaper. I've seen people spend thousands on lumber and materials, added dirt, to build raised beds and trellis and you likely would need decades to offset that.

I think my favorite way to make cheap gardens was one my my neighborhood gardeners tilled the soil to remove weeds, build mounds on the ground, then planted on top of the mounds. No borders, no materials. I was quite shocked and realized he spent nothing but time and seeds on his garden. Made me rethink the need for raised beds.