What chainrings are compatible with the Dura Ace 7900 crankset (apart from 7900 chainrings)? I need to get a 52t chainring for my 7900 Dura Ace crankset but am not willing to spend 150 euro on a chainring. Would an Ultegra 52t 6700 chainring work on the 7900 Dura Ace crankset ?
You need to check on three things to ensure that your new chainrings will work:


The number of arms on your spider. Road bikes typically have five.
The bolt spacing. This is most commonly expressed in BCD, or Bolt Center Diameter. The idea of BCD is a simple one, it's the distance from the center of a crankbolt to the crankbolt immediately opposite it on the crank. However, this is not necessarily an easy thing to measure. For one, the crank is not flat and it's difficult to measure across the bulge in the middle. Further, on a five arm spider the measurement has to be taken from an imaginary bolt since no bolt has a direct opposite. However, Sheldon Brown comes to the rescue as usual. His site has an explanation of how to find the BCD measurement with a ruler and a bit of math. 
It is also a good idea to consider is the original chain compatibility (compatible with 9-speed or 10-speed). 10-speed chainrings should be used on a 10-speed drivetrain, but you can probably still get away with using 9-speed stuff in most cases. The chainrings may vary in width slightly due to slightly narrower chains on 10-speed drivetrains. It's rarely a problem using a ring that is too narrow on a larger chain, however you probably couldn't use an chainring intended for an 8-speed drivetrain on 10-speed drivetrain.