Start with a 300 (or even 150)-gallonRubbermaid stock tank. Cost c. $150.00 Add a fairly powerful, but not outrageously expensive, filter/fountain from Wally world. Cost c. $50.00 Set the tub in the ground as far as suits your taste. Mine is only buried as deep as the bottom ring to keep dogs from falling in and to make it easy to work on. You could bury it deeper to show less of the sides, but this is not a big issue if you disguise it with plants.
Add water. If you live in town treat the chlorine. Aqua Safe works....cost about $8.00
Add plants.... you can get a box of water lilies for about six dollars. They sit on the bottom. You can get hornwort, curly pond weed and any number of native or imported plants from any wild pond. They are free and they add oxygen and filter impurities just fine. They will root themselves or just float around. Some are invasive so be careful with them. You can also buy water lettuce and fancier stuff pretty cheaply. (Cobleskill Agway has the best water lettuce around here; Lowes has the best cheap stuff for the edges...corkscrew rush, sweet flag, little cat tails... things like that.) You can make a plain old canna corm into a water canna by starting it in a pot and once it has leaves and roots setting it in your pond on blocks or in a hanger, partly submerged, for a delightful and inexpensive water garden plant. You can grow watercress in a pot partly sunken into the pond too. Experiment!Many common potted plants like wet feet. You can put plants in the little plastic-covered, metal flowerpot hangers the dollar store sells. They will hang on the side of the tub very nicely. Or you can set cinder blocks or bricks in the bottom of your pond and put plants on them.
Next plant whatever already grows in your yard around the base. Set house plants that will take the hot sun on rocks and bricks around the outside for a layered look. Throw in the cheapest gold fish you can find. Or a couple of minnows from the wild pond. Even guppies will summer in the garden pond as long as you bring them in when it gets cold. They will also procreate like mad and take on fabulous colors as the pond offers whatever they need to thrive. You can add other critters as well. Crayfish maybe? Snails from the pond? Frogs will come on their own and offer much entertainment.
Our pond is three years old and we have had a good time with it. I spent well under $500 bucks in the complete set up and it is much bigger than the little pre-formed ones you can find. The key to having what you like, I think, is to be creative in what you plants and the animals you add.