check it out:
So, much of the answer to your question isn't with what the ARV is, but with what the list price is. And a good local Realtor who specializes in foreclosures will know the policies and practices of the lender: How often it drops the price and by how much, and how much under list the lender will accept.
Taking my example from above, if the BPO came in at $80,000 and the lender drops the price by $5,000 every 45 days, after 90 days it might end up priced around $70,000. Then, many lenders have a range they'll consider; for some it's in the ballpark of 85% of the current list price. Well, 85% of $70,000 is $59,500.
So, it may be possible to find a property with an ARV of $100,000 and make an offer of around $60,000. It'll take some patience and a good Realtor who knows both what the lender is likely to do and who can tell you what the ARV on the property is likely to be.
Hope that helps.