You do not have to buy a real trellis to take advantage of upward growth. Planting peas, some squash varieties and other climbing plants along a fence will accomplish the same thing. Tomatoes can be supported on a trellis or fence as well, instead of slumping over and using more space than they need on the ground.
One inexpensive and extremely sturdy option is to purchase livestock panels from your local feed store and form them into arches along your walkways. A typical livestock panel is 16 feet long by four feet high, and when you fold it over into an arch, creates the perfect height to walk through. Climbing flowers, vegetables and other plants will grow up and over the panel, giving you a living archway.
Those same panels can be stood upright, as they are meant to be, and used in place of fences or other trellis options anywhere you would like. Pound a metal fence post or two into the ground to support them, and they will easily bear the weight of even your most abundantly producing vegetables.
By creatively utilizing space not otherwise needed in your garden, you can get more use out of smaller spaces, allowing you to do less tilling and amending for the same great yields as larger gardens.