A few years ago, I thought it was a good idea building a pond/water feature. I estimated about $400.
When it was finished it ended up costing over $7k.
I made mine from stone and then fiber-glassed the inside.
But as mentioned, its whats underground that counts.
Unless you want a breeding ground for mozzies, you have to do it right.
I have some Koi fish in mine. They're not there for looks, they're there to help keep the mozzie count down(works well).
Pumps/filters/UV sterilisers...its all needed.
Build yourself a little rock garden(cheaper running costs) ;)
Good luck
Mick