Welcome! I'm sure your GSDs are happy you've made the switch to a raw diet. I know it can be a bit worrisome trying to make sure you're providing the right portions and types of food for them, but the longer you're at it, the easier it becomes.
Re: question 1-- there is no real easy way other than "eye balling" it for a rough estimate.
Re: Q 2 -- Beef heart would be good to balance out the bone in the chicken quarters.
Re: Q3 -- If you're not feeding a lot of grassfed meat (most people don't), then offer fish oil. I have salmon oil capsules that I offer mine; they like to eat them like treats (the cat gets one, the dog (Great Pyrenees) gets about 6).
Re: Q4 -- budget depends on what is available in your area, what kind of meat you're willing to feed (i.e. 3D meat; human-grade), and nutritional/caloric needs of the dogs. If you hunt, buy live animals and slaughter them yourself, and/or join a co-op, then you can keep costs lower. If you're into human-grade/organic/store-bought, then your costs will run much higher. Your geographic location also affects availability and cost. I would estimate about $100 per month for both dogs, but it can run lower and higher.
Here's my general "getting started" advice:
PMR ratios are 80% muscle meat/10% bone/10% organ. You may need to adjust these ratios slightly as each dog’s needs differ to some degree. Some people feed “balance over time” (i.e. over the course of a week), whereas others balance out the correct ratio for each meal.
At least three meats in the rotation--but more is better. Red meats are also supposed to be more nutritious. Start with one protein (whatever is readily available) and add another when you feel your dog is ready (e.g. every other day or week).
Make sure to give liver at around 5% of diet (either feed weekly or daily). This can be from any species. You may offer small amounts (e.g. bite-sized pieces) from the start to gradually build your dog up to the recommend amount.
Chicken feet (and duck, if you can find them) are a good source of glucosamine and chondroitin (for joint health).
For conventionally raised meat (like you get at the grocery store), offer some salmon oil or oily fish (e.g. mackerel) to the diet for omega-3 fatty acid to help balance the omega-6. Too much omega-6 in the diet can cause inflammation, hence the need to balance it. Grass-fed meat animals have a better omega-3/omega-6 ratio which shouldn't require supplementation.
Work Wonders by Dr. Tom Lonsdale
Raw and Natural Nutrition for Dogs by Lew Olson, PhD
Canine Nutrigenomics by Dr. Jean Dodds