As stated below - what they paid for the property has no bearing on what they will ultimately list it for.
Your offer should be based on two things:
1. The amount that comparable properties in the area have been selling for in the last 6 months.
2. The condition of the property and how much it's going to cost to remedy any issues.
You'll have to determine also if the condition of the property has already been factored into the list price. Buyers very often make the mistake of looking at the list price, and automatically deducting the cost of fixing any issues they see. If the "homework" has been done beforehand however, you will often see that Fannie Mae typically prices their properties competitively from the start - already factoring in condition issues.
Additionally, Fannie Mae markets select properties under a program called Homepath. To make an offer on one, you'll have to find an agent and have them submit an offer through their website. Every situation is different, but Fannie Mae generally highly trusts the valuation opinion of the listing agents they hire to market these properties. If you try to lowball them, they will likely either outright reject, or ignore your offer completely - especially in the first 30-60 days. If the properties don't sell, as the market time increases to 90-120 days, you will typically see strategic price drops.
More often than not however, these properties are priced very well off the bat, and typically you'll see them sell from anywhere from 92-100% or more of list price (especially if there are multiple offer rounds). Hire a local agent who has a good grasp of the market to advise you accordingly.
Hope that helps!