The Archer posters have several things going for them:
1. They are colored to-scale. Anyone can reproduce period posters (I just bought a MiniArt Norman city building with French posters included, but the colors are neon-bright.
2. They are printed on a very high-quality paper. That might sound like a stupid idea until you realize, again, that scale posters would not have the large pores and defects on your average laser printer paper.
How cost-effective are they? That depends. Any after-market product is likely something you can do yourself. Heck, I have a friend with a metal lathe who is making his own gun barrels. I'll pass on that, and gladly give my money to RB Model.