Hi Rainbowfish,
Put very simply, when drugs are developed the manufacturers take out a petent that allows them exclusive production rights. Metformin was originally developed back in 1979 and the patent has long since expired, making the drug "generic", i.e. can be produced by anyone which keeps the price right down. Glucophage was developed in 1995 by Bristol-Myers Squibb, but the long acting version (Glucophage SR) was only developed in 2004 so is still under patent. This makes it much more expensive than the generic metformin, which is why GPs tend to only prescribe it when specifically asked.