My pond hasnt been a money pit ... the slate I got to make the waterfall was "recycled" from someone elses garden (they were sooo happy to have someone cart it off) ... my pond has two deep portions (so my waterlilies and fish can survive the winter) made from > 1) a huge ole plastic cement mixng tub and 2) a corner plastic bathtub that did not fit into my new house > both of which I buried over 6 foot deep and then dug out the form of the rest of the pond and lined it (didnt bother with sand) but used old carpet from the old house underneath pondliner plastic > I really suggest you go with the pond liner > it will last longer than the heavy duty plastic (which in my experience was not strong enough withstand pond substrate) ...
It is pretty "self sufficient" (I have not had to feed my goldfish for several years now and some are now over ten inches long > I started out with five little bitty 35 cent goldfish from the petstore and have never had to buy more) > I rarely have my pump on (took it out almost four years ago to clean and just recently last month got around to that (redid/cleaned my little waterfall and all the leaves and such that had fallen in the cracks). I get visits from the occasional heron (mainly in the spring) and I figure that is what keeps my goldfish and frogs from overpopulating my rather small pond (I have a fairly stable resident population of around twenty goldfish >nine are between 5 and ten inches)
I know many with ponds and they happily donated plants when I first started it (now we trade your plants will quickly outgrow your pond and you need to keep this in check at a certain point (sell/trade or use for the compost heap)...the iris (pond /bog variety) are the tall stalks you see in the foreground are winter hardy and flower for almost six weeks but I do not yet have a pic to put up of them in flower (these pics are a couple years old)... I do have one of my lilies:
since these pics were taken I have added a few things (all of which flower)... the plants need a bit of sunshine (altho too much will cause algae problems , if in total shade you will have problems). The waterlilies leaves and wanter gentian (an oxygen plant that is fab for "covering" the water and providing shade underneath this mat of green leaves that have a similar shape to the lily leaves only smaller) has beautiful yellow flowers (sorry no pic)
and here is one of my pond froggies in his favorite place (when I have my "waterfall" pump on):
...and here is a pic of a bird that likes to bathe there (happens to be one I rescued and it stuck around but that is another story):
In short, if you dig a couple DEEP places and put tub or whatever in there >I have ther surrounding area at different fairly shallow depths for different water plants and a bog surround about a third of it (see the earlier posted link from Daves garden on "bog filters) and plant a lily in the deep tub part (fill with substrate and pond/lily ground/dirt from garden center) > use an appropriate lily for the depth you have (I stole mine from the river near me
) AND plant appropriately (choose plants that are good at feeding on the nitrogen created by your goldfish poo for instance and lots of plants that provide oxygen) then you wont have to be dependent on a pump and filters and have to clean your pond out (I have had mine for almost ten years now and never "clean" it > my water is clear and I can see my fish just fine !)
The link someone gave you earlier to davesgarden has great info (wish I had had that info when I first started out with ponds lol)