I like 'Loser pays the lowest amount either side paid'. In other words, if big corp spends $1,000,000 suing citizen A, who spends $5,000 for his defense, and Citizen A loses, he pays $5,000 of big corps bills. If it's reversed, same applies.
That would discourage massively disparate spending in court, while still protecting the innocent target of lawsuits to some degree.
The problem, I guess, is that could encourage patent trolls to simply hold down their own expenses while attempting to incur massive expenses on the plaintif. The idea is to discourage the "lottery mentality" of our current system, where patent trolls are attacking giant corporations in the hopes of a massive settlement or patent licensing fee. They're currently safe because their own expenses are relatively fixed (with retained lawyers on staff, or lawyers paid on commission - only if they win), while the defendant may be forced to spent a lot on their defense simply because of what's at state, or because of what they're being asked to prove.
Dryeo's answer probably makes the most sense here - leave it to the discretion of the judge. If they felt that the suit was brought without merit, they can order the plaintiff to pay 100% of the defendant's bills. If the suit was felt to be merited, then perhaps set at some minimum threshold, like what happens with criminal sentences.
It's not perfect, since it's still subject to the same sort of abuses by "specialty" judges. That, incidentally, is something that's may need to change as well - plaintiffs shouldn't be allowed to pick their private little courtroom to sue from. We see the abuse there as well. If you forced plaintiffs to sue in the home courts of the defendants, it might not be fair, as the judge may well be influenced by a powerful industry and act as its protector. Well, as it turns out, it's not fucking fair the other way either, as patent troll have built a nice little industry around judges that tend to favor their practices. Maybe for interstate disputes we should have to use some sort of lottery system, so these cases are randomly assigned to the territory of a disinterested 3rd party.
I don't know... things just get messy whenever lawyers are involved, because it's their damned jobs to find the maximum advantage within the framework of rules, and they'll keep doing so even if the rules change. It's sort of hard to fix a system like that, but for all we rant at lawyers, it's still better than the alternative. That is, it means that we're a society that's based on the idea, if not always the perfect execution, of the rule of law, and not the simple exercising of power.