My son uses a Graco Cozy Bed model 8840WW (this is important because several other similarly named models were recalled), it's a plastic sided bed that uses a standard crib sized matress and sits very low to the ground. He's used it since he was about the same age as your child and he's now almost 5 and just about to outgrow it.
These guys appear to have it for $69. It's basically indistructable and did a nice job of keeping him off the floor despite following in his dad's footsteps of being a heavy tosser in his sleep =)