Bearing cream is already pretty cheap, but you can do it cheaper (and better!).
How to clean your bearings? If you use oil and soap it will failed. It doesn't have to be expensive, all you need is gasoline. Yes, gasoline.
First, filled a small circular container with about an inch of gas. I placed the bearings in the gasoline with the covers taken off. Then for about 10 minutes, spin the bearings in the container. The dirt will accumulate into the middle. Poke a stick in one of the bearings and sort of stir them, that's why you need a circular container. You'll see the dirt floating sooner or later.
Just let them soak overnight. Don't leave it in a warm area condition, I've never experienced a fire, but it makes me sound responsible saying that.
By the time you realize your skateboard isn't put together, get your super bearings and take them out of the gas and dry them with some old rags.
Get a stick or pencil, and spin the bearing on the stick. If it barely moves, stir it in the gasoline for a few minutes, because the dirt probably got back in it.
Now, just put your covers back on, and put your bearings in your wheels. Enjoy the feel of the smooth, fast bearings!