Cost depends a bit on how good you are at finding freebies, and how well set up you are for making the mods to the kiddie pool, and digging a hole.
I had to add the smiley, because digging a hole in my yard is QUITE the undertaking. Dig one inch, hit a rock, pry it out (could be 4 ounces, could be 100 pounds). If you can do this yourself or get some other very kind person to do it for you, it saves money.
Also, remember that for the pool to drain, the drain hole has to be higher than the outlet. No problem in my case - I live on a hillside, and decided (after much consideration), to place the duck pen and pool fairly high on the landscape, so most of the gardens are down-gradient of the pool. I plan to siphon the water out of the pool and onto the gardens or compost piles.
Another idea I have is to use a hand-powered bilge-pump to empty the pool. That will run $25 to $35. I may pump out all but the bottom few inches, and bucket the sludge out by hand.
Fittings for a drain will probably run less than $10 each, unless prices spike up. You'll need a drill with a bit that makes holes, and a steady hand, because if you push too hard, you can crack the plastic. I use rubber gaskets inside and out, some people use silicon caulking. The tools cost more than the parts, in my experience. But the tools can be used again. Drills are 50 to 250 dollars, depending on your level of fancy. I have a simple bit that makes holes but works best in really thick, flexible plastic (like food drums). I don't know that it would work with kiddie pool plastic, which is thinner and more brittle. If you get a nice keyhole drill, that can set you back $30 or so. Again, it can be reused.
Dave Holderread suggests no less than three square feet per bird for their shelter (I remember that number clearly), and I think no less than 10 square feet per bird for their pen (need to check that).
In my stony soil, my plan is to try to dig down about five inches, and make up the difference (10 inch deep fiberglass pond liner) by piling soil and stone up around the edges.
To keep the birds from getting too hot in the summer, I have placed their shelter and pool (in my design, so far) under a sugar maple, which means I will be cleaning samaras (maple seeds) and leaves out of the pool. Okay. Part of the deal.
I plan to have a night pen, 4' x 8', wrapped top to bottom in half inch hardware cloth, and a 4' x 8' shelter, 4' high, which I am building from plywood. The roof will be sun-tuff over hardware cloth on a frame of 2" x 4"s.
