For your sake, I hope you have insurance on your phone! If not, I think sprint charges like $119 for repairs - but don't take that to the bank. I would take it to your nearest service center tomorrow, and see what they say. However, even if you have insurance, there's still a copay.
As for safety for using it while it's cracked - i have no idea - I've not cracked mine (yet! lol, I have six dogs -- I'm amazed I've made it this far!)...Good luck!
You can find a replacement screen on Ebay. It is only 29 dollars. There is a site that give explicit directions on how to replace the screen. The repair takes about 2 hrs. It is tough but if you are good with your hands you can do it. If you try it be careful and take your time. I am not a professional but I am very good at following instructions and you must be.