Loan Items
I all depends what the part is & wether it`s a "POOL PART"
Most airlines are in what is known as a pool for AOG (aircraft on ground) items The general rate is/was 10% of the value of the part. If for instance you borrowed a valve for your aircraft and you didn`t manage for what ever reason to return it to the lender within 7 days, then you would buy it!! The big thing with pool parts is that if you are the suppling airline, such as BA at LHR, and another airline wanted a bit that you didn`t have in stock then you would have to take one off of your aircraft to satisfy the demand!!
If the part was not a pool part then in all probility you would not borrow it as, you are quite correct in saying, it can be very expensive with a down payment plus a hire fee. Again the best way is to purchase it outright.
Tristar500