Treated wood also costs like twice as much. I know that as a guy trying to just plop together a garden box I'd probably go for the cheapest wood that doesn't look like it will fall apart right away.
Treated pine is cheap here in Australia though the treatment is not exactly proven food safe.
Untreated will not last. Treated will potentially release chemicals into your soil/food, though you can line the interior of your bed with gardening plastic.
There are also slightly more expensive options for treated wood (branded EcoWood and so on) that are a good option.
(I have seven 1.2x2.4m raised beds in my garden. Bring in fresh soil otherwise you will get endless cycles of weeds - that's still one of my main problems two years in.)
Seems a bit wasteful to me, of the wood and time. I'd rather pay a little bit more and build the thing once rather than be patching it up or replacing later.
Untreated pine is also a risk when it comes to termites.
