Apple Mac OS X Lion: the cost and release date At which date will MAC OS X Lion be released?

And if I have Mac OS X Snow Leopard, can I upgrade to Lion for free?
There's no way to know that right now. Apple does not disclose such information.

Wait for Apple's June event where they might specify the launch date.

One thing is for sure, you'll have to pay for it, there won't be a free migration path from Snow Leopard, unless you bought a Mac computer the week before Lion's release.