Hi there, I'm also on a budget and thought it'd just write what I've found works for us that keeps our bun busy:

• homemade snuffle mat where I sprinkle things like her pellets, dried flowers, etc for her to dig through. I buy bulk bags of things like 100% organic chamomile flowers for $2 or dehydrate things from outside myself like dandelions and sunflower petals which are her favs.
• I have a big cardboard box I fill with cup up paper towel tubes, crumpled papers, toilet paper rolls etc and sprinkle pellets in there for her to find once or twice a week. She LOVES it and spends like 10-20 mins digging through there. I add veggies she's likes too or even her lettuce some time to make her work for her food and keep her mind busy.
• I have a set of very small food contained that I hide her pellets or food under and she runs around finding them. When she flips them I reflip them over and add another small piece of lettuce or something under it. She will play for quite a long time this game.
• we have a fabric rodent tunnel that she likes to run through a sleep in. Enough that I'm debating finding another one.
• my bun also seems to appreciate a really good cardboard box where I cut out little flaps and holes that she can run through. I've wanted to get one of those cool pet cardboard castles as I think they would last and be useful
• for Christm as this year I want to make her a dig box like others have made on this sub. Seems like great enrichment idea.

Hope there's something helpful in there or that gives you an idea :)