This is an appropriate situation for some basic game-theoretical analysis.
First, you should ignore anyone advising you to not worry about it because you're unlikely to get sued unless you're successful. This is absolutely false. You can and will be sued if you present a strategic threat to someone. The case of FlightPrep vs RunwayFinder is typical.
Despite the fact that RunwayFinder had no money whatsoever and the author gave away his hobby project software free of charge, FlightPrep still sued him out of business.
RunwayFinder did try to fight back, btw, but found the costs staggering, even though donations were solicited to help defray legal expenses. If you care to see how that strategy actually plays out in the real world, consult Google, it's well documented.
Assertions on this topic that begin citing probabilities or risk analysis are entirely bogus. First, what methodological process was used to collect the data that any such risk-based analysis is founded on? Most companies and products patent trolls destroy never generate any court records because things never get that far. The majority of companies that have met their fate at the hands of trolls leave no official record to be accumulated into a meaningful statistic.
Consider also that trolls engage in mass threats via attorney letters. What are you going to do if you get one of these? Ignore it? In fact you'll compelled to answer it, which is to say you'll be compelled to start spending money on lawyers. If a lawsuit is merely filed, a relatively cheap thing to do, in some far flung courthouse- typically in the Eastern district of Texas :
you'll fold because while the other side may or may not be ready to take you to trial, you know you can't afford to defend yourself in some far distant state having been served with documents you can barely read.
By similar analysis, as soon as a troll gets a reputation for going after small developers, all other small developers will be forced to take them seriously- a fact that isn't lost to trolls. This is similar to a drug dealer capping, say, a ten year old he finds selling on his turf. The 10 year old wasn't a real threat and had no business to speak of, but all the other drug dealers now take him seriously.
Just as the possible threat of a lawsuit is sufficient to make you think twice about entering the market, the mere sight of just one dev in your product space being sued is enough to make anyone fold. Those who stay must be staying because they're making money and now the troll knows who to go after in earnest.
If you lose a patent lawsuit in the US, you also lose your rights to sell that software anywhere the US has the typical international IP agreement with because while a country may not recognize foreign software patents, they all agree to enforce any IP judgments issuing from a reciprocal nation's jurisdiction. So if you're sued in the US, you can't run to the EU at a future time with your software and start making money, you effectively lost that right when you were sued in the US for selling a product which infringes by US law. IN the eyes of the law, your software is someone else's property and you're trying to sell the stolen goods in the EU. The EU authorities are obliged to pursue.
One other note. The US Supreme Court decided in 2007 that creating software in the US and shipping a "golden master disk" to another nation's jurisdiction did NOT constitute infringement if the software was never sold here:
So it appears to be safe to write software here and then sell it there.
What game theoretical analysis suggests to us then is, just avoid the US market. Avoiding the US market for most app developers has no real downside. Although theoretical profits are left on the table, if your app was going to make you a millionaire in the US market, it's probably going to do the same for you in the combined markets of EU NZ and other developer friendly markets.
Publishing software in the US is a kind of Russian roulette. If you care about your future, then you don't play. If you're immune to fear of death for some reason, then by all means, pull the trigger. There is just no economic reason to conduct business in the US given the finality of a judgment against you. In effect, the threat of a lawsuit issuing from any old IP lawyer functions just like a letter from some medieval king before the Magna Carta, decreeing you to be an enemy of the crown. There will be no trial. There will be no appeal. You're just dead, 'cause you are, 'cause someone (the troll) said something to someone (the court). Just cuz.
You can make all the money you need and more selling into the EU, NZ and other developer-friendly nations.