depends rather on price.. c-c group sizes at 30 yards I'd say the below are acceptable...

£250 - 20mm (my very cheap AGS PCR-1 does about 15mm)
£450 - 10mm (my old S400 did way better)
£700 - 7mm (my Phoenix does about this and so does my Rapid)
£1200+ - 5mm (never paid that mutch, but my old pro target was about this level of accuracy - modern equivalent AA FTP ?)

Don't get me wrong, I may expect better, but if a new gun at those prices can't match that it'd be going back. I have a couple of (very well tuned) springers that will shoot 6mm groups at 30 yards all day long.
Chap at our club sent a £370 PCP back as it was shooting about 20mm c-c at 30 yards - they accepted it was faulty and refunded his money.