I have a very small pond that's roughly 4' x 3' x a foot deep. It's lined with a thick layer of non-woven felt padding covered with butyl liner. I can't remember how much that was as it was 15y ago but it was definitely less than £100. I had previously had a bucket pond, with a miniature yellow waterlily in. They are £20-25 each. That got moved over, has been split a few times and is still going... so though they can be expensive they are very long-lived. Within the first few months the frogs moved in and then newts... despite the goldfish I'd got to keep the mozzies down! The most expensive part was all the stones I bought for disguising the butyl round the edge.
I never bought a filter or fountain as waterlilies don't like moving
water. The downside of that is I do have to net to stop leaves falling
in and rotting in autumn (more £), and even with that, I end up having
to scoop netfuls of goop out of the bottom. I've cleaned the whole thing
out and refilled just once because I was a novice and hadn't realised
how stagnant they get if you don't have a filter and keep the leaves
etc. out. Yuk!
I don't have fish any more, though, as my username will attest! I scoop out the mozzie larvae with a kid's fishing net. It's currently full of tadpoles and I've seen 4 newts at the same time so there's at least that number in there. Birds bath in it, drink from it, so do the cats, and squirrels. I've had damselflies, dragonflies and semaphore flies. The lilies are just leafing up. I love my little pond.
Ongoing costs are a few quid a year for anti-algae barley straw extract to stop it going pea green and also the stuff you need to put into tap water to make it safe for pondlife (if you aren't prepared to let buckets of water stand a couple of days for the chlorine to escape). And time.