If you REALLY cannot afford any mats, I would agree that dirt with shavings OVER the cement would work. My barn has cement floors and I use (4' x 6') 3/4 inch rubber mats, 4/stall, ~$40/each, so each stall cost me $120 . MY horses have most of the year turnout, and I only force them into their stalls--like I have to force them?!?--when there is ice. I know that the full-stall mats are upwards to >$300--is this the price you are referring to? Honestly, the 3/4 inch mats are hard ENOUGH to move and clean when the stalls aren't in use--I fold them in two and drag them, and it's an all day job/stall--so I have also double-uped on (4' x 6") 1/2 inch cattle mats with buttons, which I also use in my trailer and that has a wooden floor. No slippage like some mats that are too smooth can cause.
I think the 1/2 inch cattle mats are cheaper--you could lay down dirt and use those--IMHO, rubber mats are NEVER a waste of money.
I have heard of wooden floors in stalls, before. I think the places that use them have drainage underneath the stalls. I am not sure that your horse won't slip on the wood, either.
BTW, when I am NOT trailering, my mats are cleaned and they line the walkway in my barn so my horses really don't walk on any cement. Even the old ones with cracks get recycled under heavy use gates. (I also use the worst wear ones for my chickens.)