I am also an IBM employee. I have sort of the same numbers, however slight different. I am almost certain it is $1,500 for your first file. Then, for subsequent patent files, the price depends on the number of inventors for the patent. If you have 4 or less inventors, you receive $750 or $800 per inventor. If it involves more than 4 inventors, you receive $3000 divided amongst all of the inventors.
Also, you receive patent 'points' as you file. The first and subsequent files are 3 points each. Once you hit 12 points, which is the plateau, you hit $1,200.
IBM is a very very patent driven company. They have internal patent competition within the company. Hence, you promote internal competitiveness within your teammates, while also placing the entire company forward. IBM is a great company to work for if you want to work on cutting edge technology. I am not being biased here, but I've worked for Lockheed Martin and a few aircraft companies... and IBM thus far as been the most rewarding in the technical sense.