Well. I AM a mortgage guru...and although Gray is correct that buying points is paying interest up front, it CAN have some distinct advantages. It totally depends on two things. One, what will it cost on the day you lock, and two, how long will you have the mortgage? Points typically see their costs recovered in approximately 3 years. Any more than that and you're paying too much for what you receive. So after 3 years, you are money ahead. But if you only plan on having the mortgage for a few short years, you don't gain anything. Another issue to consider is points are deductible on your taxes. Therefore, if you are in need of this tax advantage in the year you buy your home, it is an additional benefit. This however is NOT the case on a refinance. Those points must be amoratized over the length of the loan. See your CPA for proper advice.
In your question above, I THINK you're looking for the APR if I understand your question correctly. This is Reg Z...also noted as a Truth in Lending. It isn't the best document in the world due to it's manupulative abilities by unscroupulous lenders. However, it does give you a direct relationship of costs vs interest rate. Your 5% of the loan paid up front makes no sense to me whatsoever. Are you saying you're putting 5% down on the house? That's not prepaying the loan. That's your down payment.
By the way, your interest rate IS YOUR INTEREST RATE!! If you get a 5% rate, then that's what it is for as long as the term you agreed to. No amount of fees and points can change that. However, the APR is an indicator of what that interest rate is going to cost you...and it is not necessarily over the length of the loan. Let me see if I can explain it to you like this...
If you buy a TV from me for $1000 and I agree to finance 100% of it for you for one year at 5%, at the end of that year, you will have paid me $1050. Your interest rate and your APR are both 5%. But...if I were to charge you $25 to come out and set that TV up when you bought it, then at the end of the year, you would have paid me $1000 for the TV, $50 interest, and $25 set up fee for a total of $1075. Therefore, your interest rate would have been 5%, but your APR would be 7.5%. As you can see, APR and interest rate do not necessarily have anything to do with each other than to show you costs involved with the transaction. Where is should make you take notice is when the APR is significantly higher than the interest rate. In that case, I would want to know why and look closely at what's going on.