Rather than spending money on buying more plants, try out some simple techniques to make more plants from the ones you already have. Think of it as "cloning" your plants. Some plants are easier to propagate than others, so start with the easy ones and graduate to more complex techniques as you get more confident.
Some plants, such as philodendron or lucky bamboo, will promptly put out roots in just water. Snip off a section of the plant that includes at least two joints, strip off all but the top leaf or two, and stick the ends in a jar of water. Within a week you should start to see a few straggly roots starting to grow from the bottom. In another week, you can plant the new shoot. Make a hole in a fresh pot of potting soil and gently push in the new shoot, taking care not to damage the new tender roots. Keep moist but not soggy. When you see new leaves at the top of the plant you'll know that your "clone" is taking hold! When you are propagating a vine such as philodendron, you can cut a very long piece of vine into several smaller sections and root each one; just be sure that each section contains at least one leaf nodule; this is where the new roots will emerge.
Some plants, amazingly, will reproduce from a single leaf. A good example is begonias. Take a healthy leaf from a healthy plant and push it into a new pot with fresh potting soil or peat moss. Push soil around the leaf to remove air pockets, and water well. Cover the pot with a plastic bag to improve humidity. Every day, mist the plant and replace the plastic bag until you see new growth.
For this technique you will need to purchase a small jar of rooting hormone. This is a powder that stimulates root growth on a plant's stems, and it is available in any garden center. Cut a section from the tip of a healthy branch of a shrub or even a small tree such as Abutilon or Hibiscus. Strip off the lower leaves, and dip ½ inch of the end of the stem into the rooting hormone. Tap off the excess, then make a hole in a new pot of potting soil and gently place the stem in the pot, taking care not to scrape off the rooting powder. Press soil around the stem and water lightly. Keep moist but not overly wet. It may take two or three weeks to see new growth on the plant.
Most gardeners do this every few years with plants that grow in a thick clump from underground rhizomes, such as irises or daylilies. Do this in the fall for spring- and summer-blooming plants, and in the spring for fall bloomers, so that you're performing the surgery when the plant is dormant and less stressed. Dig around the entire plant clump, being careful to cut through as few roots as possible. Shake off some of the dirt until you can easily see the roots. Separate sections of roots, making sure that each root section has some sprouts growing out of it. Trim off any dead or dying roots (they will look black and feel mushy). Then all you have to do is plant the new sections in another part of your garden and voila! -- instant new plants.
You see this method happening automatically with some viney plants such as bougainvillea, and you can speed up the process with jasmine or climbing roses. Slightly injure a branch a few inches from the tip and about 12 to 18 inches from the main stem. Then lay the injured section on the ground and hold it in place with a landscape pin or a small stake. Keep moist. New roots will sprout from the injured spot.