I'm on my third year of gardening in a community plot (today was opening day, actually!). We pay $75 for the plot, and it comes filled with compost. Water is also included (which, given Chicago water rates, is a great deal).
The organizers of my plot gave us these tips, my first year: Grow things you like to eat, and eat a lot of. Also, do not expect to feed a family of 4 from your plot. NOt everything is ready at the same time - in my area, for example, I'll be buried in greens for the next month but won't get tomatoes until midsummer at the earliest, and then only a handful. The tomatoes seem to wait to explode in fruit until it's almost time for the garden to close for the year!
Have you seen this garden planning tool?
http://www.gardeners.com/how-to/kitchen-garden-planner/kgp_home.html It is pretty awesome, if you are going to do square-foot gardening.
The first thing that jumps out at me from your expenses list is the $70 on plants. You do not need to spend that much! Seeds are much cheaper and most things are not hard to grow from seed. Some vegetables, like greens, you can just sow right into your bed - no need for seed-starting equipment. That worked fine for me last year.
I do buy some vegetables as plants when I see a good sale, or when I want to grow something that I've been unsuccessful starting from seed. (peppers, I am looking at you! They just never germinate for me.) I did splurge on a dwarf raspberry this year... but I eat a TON of raspberries and it's a perennial so as long as I don't kill it, it will generate fruit for years.
I'm not sure what sort of group you're gardening with, but mine really encourages community and there are a lot of how-to classes and events designed to help you meet other gardeners. They're active on social media, too. It's common for people to post on the Facebook page asking for things they need for the garden, or wanting to give away extra seedlings they don't have room for, etc. So that's also a great way to get stuff for free.
For me, a lot of the cost savings in gardening comes from the lack of waste. Especially with herbs that are expensive to buy. For example - I'm growing cilantro and basil because that way, when I make a recipe that calls for it, I snip off just what I need. As opposed to buying a bunch of it at the grocery store - because that's how it comes - and having a ton left over, which then turns to inedible rotten mush in my refrigerator because I forget it's there.