Do I pay $10, 000 for two dental implants when I enter the dentists office or when visit ends? If he cannot place because needs to bone graft, then?

Depends on the Dr. Every healthcare provider in private practice has his/her own financial policies....just like any other business. Some collect everything up front, some offer payment options (short/long-term), some will do half at the beginning and half at the end, some will collect from insurance first and then bill you the rest, etc. Just ask them. If you need a bone graft, the fee will definitely increase. .
Check with. You have to check with your your own dental surgeon , if additional procedures has to be done , and the total cost to restore your teeth, only your dental surgeon can answer. if you not happy with answer or can not afford see a different provider.
Financial Policy. Since each office is different and their policies are different, you should discuss the financial aspects of your care with the office manager at the office where you are being treated.