When I started I had the idea that I'd create the garden (from the existing weedy lawn and brambles) and then it would be "done."
Hahahahahahaha.
Well, maybe if I'd done a lot of hard landscaping that could have been the case but I wanted a cottage garden full of plants and wildlife. And what I didn't consider is that I might catch the gardening bug and when you catch the gardening bug, the process is actually the fun bit. And it never ends.
Agree with pp. Small plants are cheaper than big plants, seeds are cheaper than small plants. Fairs, fetes, local plant sales, FB Marketplace, local nurseries, are all cheaper than garden centres. I started by growing few annuals from seed, to see if I could, and then when I learned what perennials were I found cheap or free ones on Facebook (local groups, local gardening groups, and Marketplace) and planted them. I knew absolutely nothing when I started.
Over the last 5 or 6 years, the garden has changed beyond recognition. Compost has been the biggest expense but I am now making my own (which is easier the more plants/vegetation you have but difficult when you're just starting out.) I do still have to buy compost, but not as much. Everything gets cheaper as you go along, in some ways, because the more plants you have, the more seed you can collect, and the more they self-seed, and the more you can divide them and take cuttings to create new plants. So then you get loads of free plants! But you need plants in the first place to for that.
I used to look at full, beautiful gardens and couldn't understand how people created them from scratch, both financially and practically. For me, the answer has been "a bit at a time". Create a small bed and plant something in it. Sow a few seeds. Learn a bit about those seeds and plants and your soil and where the sun is. Repeat. Make mistakes. Understand that you can't control the weather. View failures as a learning opportunity. Enjoy seeing more wildlife visiting your garden now you have more plants. Keep going.
Don't be fooled by "influencers" that you need to buy all kinds of expensive things. You don't.
Become obsessed with the garden and have endless plans, hopes and dreams for it. This part might be optional. But I'm not sure it is.