My son was about 8 months old when he actually started CLIMBING out of his crib, even with the mattress on the lowest pegs. We decided to get him a blue race car toddler bed around 9 or 10 months. We bought it from a local thrift store for a decent price, and set it up as a surprise while he was at grandma's. He came home, and went into his room, and immediately squealed with glee. He jumped right onto it, giggling, and then ran over to me, grabbed my hand, and took me in, as if to show it to me too. That night, he went right to sleep on it, and has ever since. He is almost 2 1/2 years old now, and he still loves it. I even gave it a personal touch by painting his name and some numbers on it, and detailing the headlights and wheels with acrylic paint. Sure, they mught be costly, but think about it...they're for kids. They're fun, and you only get to be a kid once. say it's a fun AND easy way to transfer from crib to bed, and it's safe too, because the bed is so low to the ground. You can also find some with built-in toy boxes and shelves. Also, you can just use the mattress from the crib, as it fits right in. They are usually made of plastic, which makes them so easy to clean, and less chance of them hitting their heads and getting seriously injured, or getting a splinter.
Good Luck!