It's VERY important, and good for you doing your research beforehand.
This is how diet works: Staple, fruits, veggies, occasional treats. Fruits, veggies, and treats are straightforward - treats may include dried mealworms, bits of boiled chicken, bits of almond, dried superworms, yogurt, corn, etc. Things that gliders enjoy but should not be fed regularly for various reasons. Corn is on this list even though it is a vegetable because almost all gliders will eat corn before anything else, may fill up on it and not eat their other foods as well, and it has a very low Ca:P ratio. Staple is what varies; there are quite a few staples you can choose from. Original HPW, HPW Complete, BML, Priscilla Price, etc. They're all different. The only one I've heard anything negative about is Instant HPW, because it is powdered honey and eggs and many gliders do not feel satisfied after eating it. It hasn't caused harm, it simply isn't filling enough for some gliders.
I feed Original HPW + fruits + veggies each night, and sometimes a treat as well. For Original HPW and many of the diets in general, the glider gets 1 TBSP of staple each night. Regardless of diet, each glider should get 1 TBSP fresh fruit and 1 TBSP fresh veggies each night, VARIED. Variation is highly important. Frozen fruits and veggies can be used as needed, but canned are not recommended at all.
Original HPW is wombaroo powder, which is made for marsupials like gliders, bee pollen, which provides some valuable nutrients, eggs, and honey. You blend it all up then freeze it and spoon some out each night. Knowing that, here is a sample schedule (I'm assuming you're a visual person for asking that!) This is PER GLIDER. If you have two, which you should, then this is doubled. It is recommended that you use two separate bowls to discourage one from hogging all the food.
Monday:
1 TBSP Original HPW mixture
1 TBSP fruits (split up into 3. So, approx 1/3 TBSP apple, 1/3 TBSP raspberry, 1/3 TBSP banana).
1 TBSP veggies (split up into 3. So, approx 1/3 TBSP cucumber, 1/3 TBSP spinach, 1/3 TBSP carrot).
Tuesday:
1 TBSP Original HPW mixture
1 TBSP fruits (1/3 papaya, 1/3 grapefruit, 1/3 banana)
1 TBSP veggies (1/3 kale, 1/3 broccoli, 1/3 carrot)
Wednesday:
1 TBSP Original HPW mixture
1 TBSP fruits (1/3 apple, 1/3 papaya, 1/3 watermelon)
1 TBSP veggies (1/3 cucumber, 1/3 kale, 1/3 green bean)
Thursday:
1 TBSP Original HPW mixture
1 TBSP fruits (1/3 banana, 1/3 kiwi, 1/3 blackberry)
1 TBSP veggies (1/3 spinach, 1/3 carrot, 1/3 peas)
Friday:
1 TBSP Original HPW mixture
1 TBSP fruits (1/3 apple, 1/3 papaya, 1/3 raspberry)
1 TBSP veggies (1/3 carrot, 1/3 broccoli, 1/3 kale)
Basically, rotate your fruits and veggies. If you buy a pack of raspberries, you can use raspberry for a few days. A whole apple can last like a week, but you don't have to offer it every night. Just try to vary it as much as possible. If you're ever worried about calcium intake or vitamin D, give a small splash of calcium and vitamin d fortified orange juice, and it can count as a fruit. Some gliders are picky, so you might have to work with a handful of things yours like or something. It all varies. Just offer foods with close to 2:1 ratios as often as possible, like spinach, kale, papaya, raspberry. I try to do one leafy green per day only because i feel like they don't just want to eat a bunch of leaves all night... haha! Good luck.
As for your question, for two gliders, Original HPW per month would probably be $10-15. They offer a sample pack on thepamperedglider.com/supplements.htm which is the wombaroo powder and bee pollen for $6.45 plus free shipping. I would say it would last a month for two gliders, maybe even more. With that you would need to buy about $5 worth of honey and need three eggs to mix it with. If you always have eggs on hand it won't matter. A 6 pack of eggs is less than a dollar. If you buy the larger sizes, it comes out to quite a bit cheaper per month than the sample pack is. For fruits and veggies, it depends on how often you eat them. We eat tons of fruits and veggies, and eat them every day. I am always buying them. The gliders eat bits of what I buy for us. Ideally, any person eats enough fruits and veggies that most of their gliders' f/v can be shared from theirs. A bag of spinach is $2, that lasts us up to a week. A container of raspberries is $2, that lasts a couple of days. An apple is 50 cents and we use it for them for up to a week. A cucumber is 60 cents and it lasts about 3 days. A banana is like 20 cents and can be used for 2-3 days. A bag of carrots is $1 and will last about a month in the fridge, so you can feed carrots every couple of days. You can buy frozen mixed berries for about $4, keep the bag in the freezer, and rotate berries each night. You can give berry every night if you want, just rotate which one. That would last a month or two. You can get a bag of mixed veggies for $1, of which you can give them frozen peas one night, frozen green beans the next, or thaw it out under warm water, or boil a handful and store it in the fridge and feed it for a few days at a time. Mixed veggies do usually have corn, so make sure you limit corn accordingly. Gliders eat very tiny amounts, so what they eat costs very little. If you for some reason don't eat fruits and veggies, it will cost more because you need some for them every night.
All in all, for two gliders each month, I would say $12 HPW mix, and maybe like $5-10 per week for fruits and veggies each. You can buy cheaper items like bananas and cucumbers, and buy reusable items like a bag of carrots and frozen fruits, if you're concerned about cost. If you are low on money, remember to always have a vet fund available for emergencies, and/or at the very least find a vet that accepts care credit and get approved for it beforehand. If you have no money and your glider is sick, it's probably the worst parent feeling ever.
Good luck!!