Monopoly strategy if every unowned property goes to auction immediately?
With these rules, the optimal bid for properties you are NOT interested in is $1 less than the highest bid.  Even if you are highly interested a property, might not want to signal that with a very high bid.
As Toon Krijthe points out, this becomes a guessing game of what others are willing to spend.  Since you say players are running out of money, you may want to not win early properties and pickup better deals later.  You still need to bid high to help others run out of money and to disguise your strategy.
How much players should be bidding can be informed via a property value calculator like those in this article.  As the calculators point out, key variables in estimating the value are number of players, rounds of play expected, and if the other related properties are (or will be) owned.  In addition to the value for your own situation, Calculate value for the player who will benefit the most from obtaining the property being auctioned.  This will help estimate a logical high end bid for the auction.  

So key factors in determining your bid are:
 * Who has available cash to bid
 * Value of property to the players in the game
 * How aggressively/conservatively are players bidding.
 * Plans for your available cash