Without plants, the landscape becomes rather boring. No interesting places for the eye to dwell, nor any soothing scenery, not much for the soul,either. However, plants can be expensive, certainly if you are trying to establish your entire garden in one hit. Cheap plants are readily available all over the place, whether they be very young, or for that matter, very old and passed their sellability.
Sometimes, that bargain plant you are buying may be diseased, heat or water stressed, or possibly, harbouring pests. Purchasing plants or seedlings from a store, where the plants are under ordinary flourescent lights, will be doomed to failure in your garden. These plants will be weak and prone to pests and disease, due to having been under the lights. These are "soft" plants. In order for plants and especially seedlings, to be strong enough to survive outside, they need to have been exposed to the elements for at least some time. Sunlight, wind and temperature fluctuations, conditions usually found in an outdoor nursery, are all necessary for plants to be strong enough to handle being planted out. The actual planting-out places an awful lot of stress on a plant - roots get broken, soil is lost, drying-out may occur; and then getting jambed back in the ground - for starters, the soil is going to be completely different. Soils aint soils.
Nurseries are great places to find bargains, many have bargain tables full of unpopular plants, plants that they plan to discontinue carrying, or those that have become too big for their pots. Often, just because a plant is set out like this, there is nothing actually wrong with the plant, it's just that the newer stock looks better and therefore sells quicker.
The Internet is not a bad place to find plant deals, either. The ability to order online and have your plants shipped, makes for cheaper plants. This is a wonderfully inexpensive way to purchase seeds and bulbs, quite often exotic varieties unavailable anywhere else. Most online plant providers have a guarantee aginst damage, however, if you are looking for mature plants, shipping can be expensive. Most live plants will be sent bare rooted and packed in sphagnum moss to save them from drying out.
Packets of seeds, either online, or from a store or nursery, is probably the least expensive method of increasing your plant stock, with the added bonus of having plenty of left-overs for replacement of plants in the following season. The smaller the seed, the more there is in a packet. A packet of poppy seeds, for example, will have something like 500 seeds per packet. Be mindful, however, that you are not purchasing a packet "full" of seeds. Most seeds are tiny and may not even fill the bottom of the packet. This does not mean that you have been ripped off, if they were put into something comparable to their size, it would be only as big as a postage stamp, therefore easy to misplace and have little room for growing instructions. Seedlings, also another cheap alternative, generally come in punnets with 12 to 24 plants each. Sometimes these punnets have individual little pots similar to an egg carton, only smaller, and a little more expensive. In any case, punnets are a good way to start off plants. Being small they will establish a good root system over time and actually, fair better than an established plant from a pot. Being so young, new root systems will develop faster than a large potted plant.
Annual flowering plants with largish seeds, like cosmos, asters, sweet peas, nasturtiums and the like - are always better off being planted directly into the soil. These sorts of plants are ill suited to punnet growing as they need to establish their roots quickly, and don't fair well after having been pulled out of a punnet. Likely they will develop a stunted growth. On the other hand, seeds like petunias and ornamental tobacco, should always be sown into punnets, or trays first. This is because the seed is so fine and can easily be lost in a garden, they just need a little more care. Planted directly, large seeded annuals readily self sow, popping up in the most surprising places, easily establishing a "wild" garden, which ofcourse, would have to be the best way of establishing cheap plants your landscape.