His bike is a 2013 and we are in the US. I dont want to buy it, I am just trying to help him get an idea of what to ask and the slip on is in great shape, he put less than 1000 miles on the slip on
A slip-on will add virtually nothing to the value of a bike. Those things only cost a couple hundred bucks anyway.
Suggested retail value for a 2013 GSXR600 is approx. $7500. Trade in value is about $5200. If he can get $6.5-7k for it I'd say he's doing well, assuming it is in good running order and doesn't have a ton of miles on it and he has all maintenance records and tires/battery/plastics are all in good shape with minimal wear and tear.
If it needs tires, battery, maintenance etc. the value would drop substantially.
The problem with bikes like this is that there are literally dozens and dozens of them on the market at any given time so you kind of have to price them to move if you really are motivated to sell. A quick craigslist search reveals hundreds of such bikes for sale in my area.
Plenty of unrealistic mofos trying to sell some pretty old bikes for way more than they're worth.
I found an ad for a 2006 GSXR750, guy is asking $8100 for it. There is another ad right next to it for a 2007 GSXR600 for $4900.