I just bought my very first prelvoed bags. I think this is an excellent question, and one I just pondered quite a bit in my own head. I personally, had never done prelvoed before because I wanted the absolute gaurantee of being authentic, and I wanted that brand-new, nobody else has ever touched it thing happening. That said, we all change...
I really wanted a new LV bag right now, but did not have the funds for brand new. I also have always wanted to try certian bag styles out, but not enough to buy a brand new bag. Enter the vintage options.....bags that are around 20+ years old, in good condition, great prices, great patinas, a really nice option right now. I ended up with a Saumur from 1990 and a Speedy 35 from 1988. I ordered both from vintage luxury suppliers in Japan on ebay. They are both MIF (another cool thing about vintage). The insides of the bags are immaculate, the outsides are very good. I am thrilled, I have carried both already. I was really worried about them smelling weird or having black/grime on the patina. Neither has been an issue.
As for price, the original question....For regular canvas bags, I personally would not buy a used bag for more than half of the current price of the bag. In other words, if I am going to spend more than half of what the bag costs new, I am just going to buy new. For less than half of current retail price, you can get some really nice bags, in excellent condition. My bags, being vintage and in good condition, I paid about $250 each for them. I think that is more than fair. I would be willing to pay a bit more for bags in even better condition, but for more than $500, I would probably just buy new.
For other, pricier bags made by LV, like leather or limited production, the rules change a bit for me. It would totally depend on the bag, but if it is a discontinued style, I might pay close to retail for a new bag. If you can authenticate it, why not? It may be your only chance to get one.