Lowcarbveg, another cook! welcome!
I'd take my carb counts for unsweetened organic soymilk from the counts a manufacturer like WestSoy lists for its own unsweetened organic soymilk--not a perfect system, but easier than wrapping oneself around the axle trying to do a conversion without lab analysis handy. I don't have a box in front of me, but if I recall, 8 oz. of unsweetened organic soymilk comes in at something like 1g ECC (effective carbs) per cup, I'd guess that's close enough.
I'm assuming you make your soymilk from white soybeans, though...the black soybeans don't make a really good milk in my experience, and when I want dark milk I'll add cocoa.
When I made homemade soymilk light years ago, I used to just reduce some of the milk to make 'cream.' And yes, you can make yogurt from either the milk, the 'cream' or a combination of both.
As for the discrepancies between various sources for the carb counts of soybeans and the information listed by Eden on their canned organic black soybeans, Eden acknowledged awhile ago that the 1/2 cup portion listed on the label also included the canning liquid. Drained, the cooked beans run closer to 3 or 4g ECC for a half-cup, but that's still pretty reasonable.
Whether soymilk is healthier than heavy cream, half and half or dairy in general remains to be seen. Yes, heavy cream and half and half both contain significant amounts of fat, but you NEED fat to spur your system into gluconeogensis, which is the heart of why low carb eating is successful. Bottom line, a lot of veggie protein sources outside of dairy and eggs are pretty low fat, so you do need to keep a careful eye on the fats you're getting, ensuring that there is enough. You are teaching your body to learn to use different macronutrients (protein and fats) as its first preferred macronutrients in digestion, and depriving it of carbohydrates to teach that. If you only deprive your body of carbs without increasing the amounts of protein and fats it needs for ongoing metabolism, you'll only be doing half the job and will hit a wall relatively quickly.
Yes, you are carrying some extra fat you want your body to start using for fuel, but during this teaching phase when you're telling it, 'hey, pick fats and proteins first,' you have to be sure to GIVE it fats and proteins that are easy to get into the metabolic process. Once your body learns to look to protein and fats first, it'll take care of working on your body's fat reserves without skipping a beat.
As for frugality...you don't have to eat eggs or dairy in their natural forms to get the benefits of eating them. You don't have to eat fake meats (I only eat them when I'm testing a recipe, to be honest.) You can make things like seitan at home for less than a quarter of the cost of one package of pre-made seitan (one small box of vital wheat gluten makes a LOT of seitan!)
I spend my mad grocery money on nuts and seeds, myself (and good coffee, but that's another post.
) Things like nut muffins which use kefir along with ground nut flours and nut butters to create a protein and fat dense snack or meal, pancakes/crepes which use lots of eggs in the batter but don't taste like eggs when you're eating them, Karen Barnaby's cheese 'noodles,' tofu shred (noodles) from the Asian grocery store, etc. can get lots of relatively low cost proteins and good fats into your diet without making you feel like you're eating eggs and cheese all the time.
And another way of looking at frugality is: if you continued to eat in the way that got you to your current weight, how much money would you be spending on health-care related issues (if not now, soon enough?) This is money you will be SAVING with healthier low carb eating habits. That, IMO, is being truly frugal--preventing unnecessary health care expenses with healthier eating.