The SLAM was originally developed as a land attack version of the Harpoon anti-ship missile. More improvements were made and by 2000, the SLAM ER was in service. JASSM was an air force idea and the lower price of $400,000 is subject to going way up if enough of them are not built. Since the U.S. has consistently dominated the air space over battlefields, there is not a big demand for JASSM. What the troops really want are more (and much cheaper) JDAM (GPS guided) bombs. So far, the air force has the Department of Defense behind it and the navy is under a lot of pressure to support the JASSM program. The JASSM just entered service this year.
The U.S. Navy, in an effort to save some money, is trying to back out of its agreement with the air force to pay for the development of, and buy hundreds of, JASSM (Joint Air-to-Surface Standoff Missile,) The navy prefers to concentrate on it's own SLAM ER missile. Both missiles use GPS and other sensors to hit distant targets with great accuracy. But JASSM has longer range (350 kilometers versus 250 for SLAM ER), is cheaper ($400,000 versus $500,000) and has a larger warhead (1000 pounds versus 800.) SLAM ER can hit moving targets (like ships) and change targets after launched (another useful option for a missile carried by naval aircraft.) But aside from the Chinese, there is no other navy in the world that is likely to provide SLAM ER with targets. The navy has shorter range anti-ship missiles that can handle most other potential naval foes (particularly North Korea and its many smaller ships for which SLAM ER would be overkill.) But JASSM also has drawbacks. The F-18 can't land on a carrier with an unused JASSM (which weighs 2300 pounds, versus 1500 for SLAM ER).