Blizzard’s future: All you can eat Zerg rushes, no charge.

In a PC video game market that is increasingly supported by free-to-play sales models—where a base level game is given away for free and players purchase in game items, currency, or access to more of the game in small transactions—Blizzard Entertainment remains a powerful hold out in full game sales. Diablo III, World of Warcraft’s expansions released over the past four years, and StarCraft II have all sold millions of copies at $50 a pop or more. Even the best-selling PC game studio in history can’t stop progress though, and free-to-play games are going to be an inevitable part of its future. StarCraft II may be the first Blizzard game to bite the free-to-play bullet.

Speaking at a panel during the Valencia eSports Congress, StarCraft II lead designer Dustin Browder said that Blizzard is considering making at least portions of its globally popular strategy game free. “[Blizzard is] looking at free-to-play as an option for the multiplayer,” said Browder according to PCGamesN. The problem is to how to make it work.

“We don’t know how we would monetize it,” said Browder, “While it might be good fun for me to play against someone with only half the units available to them, that’s not going to be an enjoyable experience for them.”

The monumental popularity of Blizzard’s games means that the company can take its time finding the ideal way to monetize free-to-play versions of its games. Diablo III sold 3.5 million copies the first day it was on sale in May, and an additional 1.2 paying World of Warcraft players downloaded the game as well, bringing the game’s total first day population above 4 million. That’s more than Diablo II sold during its first full year on shelves. Diablo III is the fastest-selling PC game ever made. The previous record holder: 2010’s World of Warcraft: Cataclysm, which sold 3.3 million copies in 24 hours.

StarCraft II, meanwhile, sold 1.5 million copies in its first 48 hours on shelves, and an additional 1.5 million copies over the next three months in 2010. That far exceeds the sales of most full price PC games, but it’s still not up to the standard of Blizzard’s other marquee properties. With two more StarCraft games on the way, StarCraft II: Heart of the Swarm and StarCraft II: Legacy of the Void, can free-to-play bring the series’ profitability in line with Diablo and Warcraft?

The bigger question: If you make StarCraft 2 free to play, will people continue to buy the full version?