To elaborate on what Sombra posted, I suggested bombers be used as siege engines to allow all factions to potentially attack a base rather than it being limited to caps. Something like a slow moving torp that does no or very little damage to ships so attackers must chose between risking a close up attack or fire from a distance with the possibility of the torp being destroyed.
For simplicity's sake an attackers fee would probably be more managable, maybe half to 2/3rds the base's cost including modules?