I have recently purchased a gs500, I love the bike and obviously cannot resist the urge to start nodding it, I am looking at exhausts at the moment and I am not sure what to get because there are so many my budget is around €350....any pointers or recommendations will be very appreciated

Reading the Wiki is a good starting point.  http://wiki.gstwins.com/index.php?n=Upgrades.Exhaust  And keep in mind that you may also have to change jets when you install a better flowing exhaust.  http://wiki.gstwins.com/index.php?n=Upgrades.Rejetting

Another mod you may want to consider is changing the air filter.  http://wiki.gstwins.com/index.php?n=Upgrades.AirFilter  You can either keep the stock air box and install a drop in after market air filter, or remove the air box and and install a K&N lunchbox or pods or Uni filter. Beware that the induction roar might be quite loud if you remove the stock air box.

The least expansive is a slip on muffler. Aftermarket replacements are available  . Many others can be adapted from other bikes. 

For new, you might check here for mufflers and complete systems,  Then shop for price.

https://delkevic.com/motorcycles/suzuki/1991-models/gs500e-f-1989-2002/