I thought some of the synthetic lumber made for decks might be interesting.
Have you priced out that much aluminum diamond plate if you think wood is expensive...?
Do you have a lumber yard that carries hardwood nearby? you could have the lumber milled locally. Maple or cherry isn't too expensive (Maple with a dark colored truck, cherry would be gorgeous after it ages) red oak would be another inexpensive attractive hardwood choice and readily available, the shipping is what drives the price up.