Typically that's true, yes. As soon as a line of graphics cards becomes "not new," their price starts dropping. After they become rarer, the price will rise a LOT because of people wanting to buy a second one for SLI or Crossfire, and then they'll become hard to find.
If you want to buy one of the new cards, wait a few months after they're released for the other company to fire shots across the bow. For example, when AMD released the R9 290, the price of Nvidia's GTX 780 dropped like a stone by about $150 in order to retain an edge in price/performance.