pretty high i'd say. pre ejaculatory fluid DOES contain sperm, it takes one sperm to get you pregnant.
I don't know what the exact chances are, but I was regularly with one guy over the past year, and that was our only method of birth control, was to pull out.. and it went on for 3 months then in the 4th month I ended up getting pregnant by it.
It can happen. I can't say the exact chance, but if you're trying to prevent pregnancy using the pull out method, it is a very poor form of birth control. (If you want to call it birth control) While multiple ejaculations CAN decrease sperm count, it does only take one to fertilize an egg.