Classic Duke Nukem games will be removed from GOG's catalog for the new year. So they're offering the games on sale for up to 90% off (about $2.50 for all 4 games). See the quote below (emphasis added):
The Duke. Men want to be him, women want to be with him. The manliest of men, the ultimate male role model. He's been kicking ass on GOG.com for years, but on December 31 2015, Duke has to step out to get more bubble gum. On that day, all Duke Nukem games on GOG.com will be removed from our catalog, but we are in talks with the new rightsholders to bring them back as swiftly as possible. That's why we bid you a farewell for now, Duke: with every Duke Nukem game at 90% off as a bundle, or -75% individually.Get the Duke Nukem games here
With the start of the new year, Dragonshpere will no longer be a free game on GOG.com due to new publisher agreements. If you haven't claimed a copy for yourself, make sure to do so by December 31, 22:59 PM GMT - afterwards the game will be regularly priced at $5.99.
, and get the free Dragonsphere game here