I don't like buying weapons with certs, because I can't buy attachments with Station Cash, so I'd be of the opinion that no gun is really worth spending certs on. It depends on what you value your game time as though. I'll gladly pay a fiver for a weapon and then spend the 250 certs it'd cost, that probably at the rate I play and earn certs will take me 3+ hours to get the laser sight, 1x optics and then either the suppressor or advanced laser sight.