Beware...I got way wordy when typing this, but I've got lots of ideas on this topic:
Simple black beans and rice, served with tomatoes, is amazing. Or you can try coconut black beans with basmati rice, and serve that with mangoes, cilantro and avocadoes. Also, try different sauces. You can make pastas a zillion ways when you think outside of the marinara box...I like pesto sometimes, peanut sauce at others. Try adding beans to your pasta sauce. Chickpeas (garbanzos) go really well with tomato.
I like to cook rice in a saucepan, only use interesting things as the liquid. For example, I often use veg broth/tomato juice as the cooking liquid, then add pieces of tomato and some beans (black beans or kidney beans work really well here, as would lots of other stuff). You can put veg in, too. I've used carrots, onions, squash and corn. As far as spices go, that's personal preference. You can make it taste mediterranean or mexican, whatever. I like garlic, turmeric, and cumin. Then just cover and cook your rice like normal, making sure it doesn't burn on the bottom of the pan.
Couscous pilafs are super easy: put couscous, dried fruit (if you like it-try cranberries, currants, figs or chopped apricots), beans (I like chickpeas) and chopped veg in a baking pan, then top with tofu/portobello mushrooms/whatever you want (or don't...tastes great by itself.) Pour in some type of liquid (you don't need a lot) and cover the pan with foil. As you bake this, the couscous steams and the veggies get tender. Try putting fresh herbs in at the end, so their oils and scents get released. Yum!
This has nothing to do with beans, but it does feature rice: I like to make "undone" sushi...basically, make up sushi rice and put it on a plate. Then put veg on top (whatever you'd normally put into your sushi: I like carrots, cucumber, squash-all types, sweet potato, fancy mushrooms, anything that's texturally interesting) and serve it with cut-up nori sheets (or other seaweed). I like to sprinkle sesame seeds on top, and you could drizzle it with sesame oil and/or soy sauce (or shoyu, or bragg's liquid aminos) if you want. Sushi, without all of the hassle!
Try buying beans, lentils and grains in bulk. Throw barley into soups to give them more substance. Try polenta (the dry kind...the already-prepared stuff is expensive) as a base for sauces or sliced and panfried. You can puree/mash white beans and use them kind of like you would mashed potatoes. It's a nice change of pace. I also like bulghur wheat. You can dress it however you like, and it tastes good both cold and hot.
As far as lentils go, try lentil stew: I just throw some sliced celery, carrots, onion, garlic, and whatever else I want into a pot and cook them a bit in olive oil. Then I add either veg broth or water, and some simple seasonings and lastly, about 2 or 3 cups of lentils. You have to mess around with this measurement...it depends if you want things soupy or more stew-like. I boil it until it's really thick, and eat it with either rice or crusty bread. This freezes and travels well!
I've made lentil loaf before, and it was sooooo easy and good! I remember that it had walnuts, carrots, onion, rice, lentils, and other yumminess and it made 2 big loaves that were ridiculously tasty. I don't have a recipe, but you could easily find one. Also, try black bean burgers. I made those the other night and all of the prep was done in the blender...then just formed little patties and cooked them on a skillet until they were crunchy. They were delicious!
Best of luck with your budget and I'm sorry I typed so freaking much. I fed my fiance this way all through college, and we were able to save TONS of money. I also got to constantly mess around with different cooking techniques, spices and ideas, so that's why I have so many things to share with you