I'm going to run into trouble as I eat plant based (fruits & vegetables) and it's seeming pretty expensive.
I don't think there's a "should" here -- grocery costs vary significantly according to location, really. But I thought I'd challenge the idea that plant-based eating is somehow more expensive than other kinds of eating. We eat mostly plants, and a ton of them, and don't (usually) spend excessively...it's a question of learning how to dial down the costs of foods you already eat, not of denying yourself healthy options. Although it is true that this may require a bit more brainpower than some other kinds of eating...a box of mac 'n' cheese basically costs the same all year round (and definitely tastes the same all year round), whereas a kiwi fruit is going to vary considerably according to all kinds of factors! Anyway, a few thoughts.
First, we eat a lot of our veggies cooked into soups, stews, curries, casseroles, and similar forms. For these, it makes little difference to me whether I use fresh or frozen veggies...there's just not much difference in terms of texture or taste (or nutritional value, in most cases). So for a lot of the veggies we use, I can optimize costs by figuring out whether it's cheapest to use fresh, to buy frozen, or to buy fresh and then freeze (this typically only works out to be cheapest for a small number of veggies -- things with a short season, and a high cost outside of the season). I'll add that these often turn out to be our cheapest meals, by far: I did the math for my basic lentil soup, and it works out to be about $0.40/serving (in US$).
Second, there are plenty of veggies/fruits that I prefer fresh/unfrozen. Some of these are highly seasonal, others store pretty well and tend to be cheap all year around (potatoes and cabbage are obvious examples). So if I want to save $$, I need to pay attention to the seasons and to sales. Eating seasonally seems especially challenging at this time of year, so I can't claim to manage it perfectly (ahem, there's a tub of blueberries on my counter to prove my imperfection!), but making sure that
most of our meals rely on seasonal and/or stored veggies does add up to significant savings. And tastier food, of course. It also means we enjoy a tremendous amount of variety throughout the year, simply because we're adjusting dishes to incorporate slightly different veggies.
Third, planning reduces waste. One huge reason that produce seems expensive is simply because so many people don't manage to eat it all. Once I started planning meals more aggressively, I pretty much cut our food waste to nothing (minus the occasional surprise when something goes bad early, I guess). Knowing how I plan to use veggies when I buy them makes a big difference -- no more buying a half dozen avocados just because they're on sale, only to realize later in the week that we forgot to eat any. I shop once a week, and everything gets eaten, which makes a significant difference to our overall budget. Planning also means that I can alternate super-cheap meals like my lentil soup, with crunchy-but-slightly-pricier meals like salads and stir-fries, which helps us find our happy place between budget and variety.
My suggestion is to stop worrying about what's average, and start diligently tracking your own spending so you can see what your current habits are. Then start looking for the "low hanging fruit" -- places where you can make big changes in spending without really feeling it (stop buying that bag of spinach that never gets eaten, for instance, or only buy bananas at the store where they're cheaper, or whatever). After that, you can start figuring out how to tackle increasingly challenging aspects of food planning and budgeting.
Good luck!