Depends on how common they are as well as level. If it's common stuff it has to be at least 37/45 (the highest stats appraisal).
If the pokemon is also very low level it would have to be even higher to be worth the stardust investment.
Top tier stuff I would keep regardless. No point transferring them when there are pidgeys / weedles / rattatas that need transferring. When trading comes out there's probably someone out there who'll want your low level and/or bad moveset top tier pokemon.