1- What is the relation between "Satis bedeli" on a tapu and the real price that has been paid for the property?

Both of them must be same...

2- If I buy an apartment which the real cost is 100000YTL how much the "satis bedeli should be?

Both of them must be same. Never ever accept to declare the sale price at the tapu less than you'd pay to the seller. If you declare it less than the real price, you take the risk of being sued by third party creditors (alleging that the debtor has transferred the property to you to hide his property and not pay his debts, which you would definately lose in that case.) Other kind of actions are also possible but you also take the risk that after a couple of years you'd pay penalty to the tax office for the balance between the real price and the sale price you declared to the land registry. You'd not like to be sued for tax evasion, would you?