Railroad ties or cider block are inexpensive and can be used for the retaining wall as well as the stairs.
Last summer I did a wall project at my home to expand the "parking" area near the end of our drive. I used 6" landscape timbers. These run about $25 ea. and I used 20 or so. Not the cheapest option as native rock can be had for a couple hundred dollars per ton...Where the savings come in with timbers is in the ease of install. Building a wall with rock (or brick) is a bit more labor intensive.
I have seen some retaining walls built using old tires filled with soil. Tires are very cheap and when pounded full of dirt are very robust. (This is a building method used in earthship homes). One the wall is built the tires and soil can be covered with a burlap net and planted with ground cover, or add some lathe and do a stucco type finish.
There are several factors one must consider when constructing a retaining wall. While low cost is everyone's concern, you have a few things to think about.
You are saying this is a steep creek bed. A few things that raise issues are. How high is the wall going to be? And because there is water near by there are pollution considerations to be had.
The higher the wall the more strength your going to need when constructing the wall. You simply cannot just build a wall. Dead man supports that tie the wall into the ground need to be installed. The angle of the wall also needs to be considered. Not all retaining walls are vertical. Some are pitched to counter act the pressures of the soil it is holding back. A bigger issue is however is the pollution factor. You simply cannot build a wall and disturb the soils next to a water source. Regardless if its a tiny creek, or a larger body of water. The EPA is very strict when it comes to doing work next to water. Even something as simple as installing a set of stairs. You need to consider what will happen if the water rises and the stairs pull away from the ground should the water rise. All sorts of things.
Also because water is involved. No matter if the creek is dry for most of the year or not, your limited on what you can use next to the bank. Treated wood in most cases is not allowed. Nor is items such as tires, although a great suggestion. Then there is the matter of permits. No matter how much better your going to make the area around the creek, the EPA looks at it in ways you could not even imagine.
Before you do anything you need to find out exactly what your allowed to do and what permits are required. And I can tell you without a doubt, if water is nearby regardless where you live there are permits required when working next to a water source, even if its dried up or never ever even floods.
I had a client who's driveway ran through a low part on his property. And during really wet weather the driveway had water on it to a point just enough to make the ground to wet to drive on. He put down a few yards of crushed stone to provide a base so he did not sink into the ground and used it for about a year with no issues. That ended up costing him over $50,000 from fines when they found out about it. These people do not mess around. They have in most cases nothing better to do then screw around with people who cannot afford to be fined.
So please check out exactly what it is that you want to do before you end up loosing all your money and perhaps your home by simply trying to make things better.