Another cheaper alternative to the ascender is a prusic hitch and a rope.
The prusic hitch is the knot that seat of the pants uses and easily slide up and down a rope but stops cold when pulled from the business end. It's a simple and easy to tie knot. I'll try to find an illustration and post it.

PS: guys please be very careful if you use steel screw in steps. Big Country, A good friend and member here fell this year while climbing into a stand that the landowners son hung last year before dawn. A treestep pulled out only 8 feet up and he landed on his feet. For a few seconds he thought everything was OK but then he felt something running down his leg (blood).He caughta treestep on his way down and competely severed is bicep, his deltiod muscle and part of his tricep. He pulled the main artery partly out of his arm but somehow it wasnt severed (or he'd be dead). As it was he almost bled out before getting to the hospital where they gave him 4 pints of blood. Th good news is that he's healing well and should recover fully.

The point of the story is that the treestep (ameristep) hooked him only a few feet down. Something that could still happen with an ascender if it hadnt been kept very tight. If you uyse screw in steps, consider the rubber coated types like Deer Me or go to the hardware store and get some rubber caps for the end of those steps. have over a hundred Game tracker steps and they have the same sharp edge as the Ameristeps. I came across some heavy rubber end caps a few years ago in a bargain binthat are supposed to glow in the dark. They dont glow anymore but after my buddy's experience, I'm only using the steps that have the caps till I get em on the rest of my steps