[Oof sorry this is such a long post, I got carried away hahaha]
From what I understand, you feed them veggies to give them extra nutrients and things that they don't get otherwise.
For example, if you look at all the guinea pig food charts out there (best ones are the sticky on this forum and the one on guinealynx) they cannot say enough that they need veggies low in calcium and high in vitamin C.
Calcium, in abundance, can cause bladder stones in guinea pigs, which are painful and have ti be surgically removed. Also, guinea pigs cannot produce their own vitamin C, and without it, they can develop scurvy which can be life-threatening. Lots of veggies have a good balance of those things, particularly bell peppers and lettuce, which is why people consider them to be a "staple" of guinea pig veggies. And the reason people try to give so many different kinds of vegetables is not only to stave off boredom, but it's because different foods have different amounts of everything, and variety in ANY diet (guinea pig, people, whatever) is the key to good health.
Now, coming from someone who's also trying very hard to care for my little piggy bud without breaking the bank, it's actually not that hard to give them the vegetables they need.
I know some people come up with elaborate 10-different-kinds-of-veggies-a-day diets for their pigs, but let's the reasonable, not everyone has the time or money to keep up with that schedule. I wish I could do something like that, but let's be honest, not everyone is a top chef when it comes to guinea pig diets, and I can't afford to spend $30 a week on veggies.
So what I started doing is I looked at the food chart (
https://www.guineapigcages.com/foru...vy-Nutrition-Charts-amp-Poisonous-Plants-List ) and chose some balanced, "feed daily" veggies that are readilly available. For example, red lettuce, cherry tomatoes, zucchini, and green/yellow bell peppers (I'm also going to try raddichio, endive, and maybe some of the "occassional" foods for variety). That's four things full of good vitamins and not too much calcium. Pricing that out, each thing was not very expensive at all. We got 2 good sized bell peppers for less than $1.50, and the lettuce and a kind of small zucchini for around a $1 each. Cherry tomatoes were around $2, maybe $2.50. That's more than a week of vegetables for about $6-7! In a month, that's around $24-28 tops, if I did my math right. Keep in mind amount of guinea pigs also affects that, and some things last longer than others. Like a zucchini might last a week and a half, two weeks if you only give little bits a day, and cherry tomatoes a bit longer if you only give 1 per pig per day, so it's most likely LESS than that. As such it's probably closer to $20-25 really, if you get certain things biweekly instead of weekly.
Then I just de-seed and cut up the peppers, cut the zucchini into little chunks, and every morning he gets a big leaf of red lettuce, 1/8 a bell pepper, one cherry tomato cut into 4ths, and a few chunks of zucchini. I serve everything up in the lettuce because the leaves are so big, and every morning he's wheeking his head off because he's just so excited for his veggies
What I give him doesn't change much, or rather doesn't change by schedule, but I still give him whatever else is in season or on hand for a bit of variety, and it's better than just one thing or nothing at all.
But basically what I'm trying to say is you can give your guinea pigs some good daily veg without breaking the bank, and it'll have lots of health benefits and all that. And if your's aren't picky like mine, you can also get a bag of carrots and feed one per pig per day, and that's a good little treat that'll last a while. Mine doesn't even try them now, he just lets them wilt and poops on them. [emoji14]
