We've spanned the creek in our backyard with a bridge. The main deck was salvaged from a nearby home - it was an outdoor wheelchair ramp no longer needed. The ramp didn't quite span our creek - 16' wide from bank-to-bank. So I build a rough support of 4x4s - also salvaged from the woods. And then constructed a small 4' deck to span the gap. All the wood is salvaged pressure-treated wood. Home Depot sells cutout lumber for .50 to $2, depending on the length & quality.
The pre-cast concrete footings were purchased at HD for about $6 each.