I have created one custom field which is Commission, over there i am trying to calculate commission. IF LeadSource is web and Amount is greter than 1000. Then commission should be 2% of amount. Other wise it should be 1% of Amount.
IF LeadSource is Phone Inquiry and Amount is greater than 50000 then the commission should be 10% of Amount. Other wise 5% of Amount.

OR(IF(AND(ISPICKVAL( LeadSource , "Phone Inquiry"),  Amount  < 50001) ,  Amount  * 10 / 100 , Amount  * 5 / 100 ),IF(AND(ISPICKVAL( LeadSource , "Web"),  Amount  < 1001) ,  Amount  * 2 / 100 , Amount  * 1 / 100 ))