A bit of risk-only pay "parts" price.
Well, of course the frame is weaker than it was. No way of telling-even with a picture-if it will drop you one day.Chances are it would fold in stages giving you some warning, but...
No one can tell you "how much" your risk is increased-not even an expert(but he would warn you off it probably). It pretty much boils down to how the little voice in your head.If it is telling you that "it" is going to constantly be worrying you, then.....
Now we can tell you how much to pay-NO MORE THAN THE PRICE OF THE PARTS-and price means the price you would pay on used parts on Ebay. The frame shouldn't enter into your calculations. Dented aluminum frames are VERY TOUGH to sell.
Now I ride on a slightly dented NRS frame, but the dents are very, very small and shallow-more "prints' than dents. I'm also old and don't do real MT riding, so the frame has an easy life.
If it is a great deal-buy it and keep an eye on it.