This has been covered extensively here in the past (Greg G., AKA “Hiking Malto” did a ton on liquid calories, including a fast PCT thru hike)…but unfortunately, it all seems to be buried and the search tool…well…
Tom K. has also talked about this quite a bit and has some interesting strategies I’ve learned quite a bit from.
In a nutshell, Perpetuem is nothing but 8:1 (by weight) carbs to protein, predominately in the form of maltodextrin and soy. And a little flavor. That’s it. I used to use it in my ultrarunning and distance cycling days but stopped because it just got too pricey. I’ve gotten back into it as it’s just so convenient. I experienced a bit of a bonk about 5 hours into a training hike the other day…half a bottle and I was good in 15 minutes.
I’m currently using Carbo Gain maltodextrin mixed with unflavored whey protein (I avoid soy products except tofu). I think enough of both for 50+ servings was about $35. The more hardcore among us (mentioned above) have been known to purchase in 25 and 50# bags.
My current mix:
“Ouzelade”
72 grams malto
9 grams whey protein
I add Tang for flavor, but not too much, about 2-2.5 tsp.
Total batch comes out to around 3 ounces and a tad under 350 calories.
Resulting batch is put in a snack sized ziplock and added (tear off the corner, it pours easily) to a 16oz. bottle and shaken w/water. Surprisingly not as thick as you’d think at this concentration and once mixed it stays mixed. I’ve experimented with a few flavoring agents, pick your poison (or none at all), but have found Tang leaves the least bottle residue and is most palatable to me (I used to use orange/vanilla Perpetuem).
Anyhow, I’m looking at like $0.70/serving, versus A LOT more for Hammer.

I’ve also moved on to DIY electrolyte supplements as well as recovery drinks.
To be honest, I could care less about eating out there these days; I’d filter feed, graze on grass, or photosynthesize if I could. Getting a good portion of my calories through liquid (about 1000/day) helps to this end, especially as I’m getting back into racking up the miles and fueling while on the move. I’m not sure how much further I could push the liquid calories…nor how much I would want to. But it really makes things easy.
In the context of this thread, I find the liquid calories to be gold in that if you’re feeling low or your stomach/appetite is off and you still have miles to do, they’re very easy to get down.