I currently have 4 bunnies. 2 which are actually mine and 2 belong to my sister that had no where else to house them.
What I have done to help reduce the cost is buy huge bags of litter. My 2 bunnies are bonded, so I only have 1 litter box for them. Then each of my sister's rabbits have their own litter boxes. I get pine litter (which I noticed many people do not like). It works well for us here. I get it from Meijer usually for $8.00 (and they have cedar litter as well for about the same price). Its super packed in the bag I get, so once its opened, there is even more in there than it looks. Walmart sells the big bags of pine and cedar too, but Meijer has proven to be cheaper more often and is closer to my house. This may throw some people off, but I only change the litter every 2 weeks. I found I was wasting tons of litter when I did it once a week because they usually go in one spot of the litter box. So, I just freshen it up after a week and move a lot of it around so I get more use. I don't recommend going over 2 weeks with changing litter though... I have done that and the smell is quite awful (as you could imagine). Changing the litter every 2 weeks though, a bag lasts me about 3 months. I think its pretty decent.
As far as pellets, I get those from Meijer too. It is their own Meijer brand and comes in a big yellow bag with the cutest little bunny on it.
It is a 25lbs bag and cost about $8.50. I have used the Walmart brand as well, which is about a dollar cheaper. I know this is a no no for most bunny owners, but I have interchanged their food between Walmart and Meijer brands, but they never mind. They eat it all the same, and haven't had any health issues from it, nor have they boycotted the other food when I have switched it suddenly. From looking at the bags, both brands are fairly similar with their ingredients and % of certain vitamins and such. The 25lbs of pellets typically lasts 2 months (my 2 bunnies have a feeder that dispenses the food for me, the other two I feed certain measurements to daily). The size of your rabbits would determine how long the food would last too. One of my sister's rabbits is a White New Zealand that weights close to 12lbs. She eats a lot.
I always buy Timothy Hay from Meijer as well. My bunnies are interesting with hay because they love it, then don't, then love it again. I typically buy the same hay and I know they like it, but sometimes they just don't eat it as quickly as other times. The White New Zealand is the only one I constantly have to fill the hay feeder for. My 2 bunnies are the weirdest with hay. Sometimes they will eat a few handfuls within 1 day, other times they will eat the same amount over a whole week! So, because of that, my hay typically lasts me for a month, maybe 2 at the most. I buy a bigger bag (3lbs I think) and it costs about $10. This is probably my biggest expense when it comes to things I buy since I have to buy it more often.
When it comes to produce, I fall short. I am not as consistent giving them lots of veggies and such. Whenever my wife and I take a big grocery trip, I will grab a couple heads of romaine lettuce that usually run $1.19 a pound, but I'm lucky if what I get lasts 5 days. So, the bunnies don't get produce often. We always have raw carrots in our house, so about once a week I give them some of those (since carrots aren't great to give on a regular basis... the vet told me it was because carrots have a lot of sugar in them). This is probably why I save more money on the bunnies.... but I also know its not the best thing to cut back on. The good thing is that all my bunnies are healthy and happy, so it hasn't had any negative effects so far (and we have had these bunnies for over 2 years).
Hopefully this helps a little bit. I know there is tons of good advice from people on here (and they probably take even better care of their rabbits than I do :nerves1). Hopefully you find a budget that will work well for you and your bunnies!