If you work out the costs of female to male seeds, say that the regular version costs 50 and the female version costs as much more as 90 break it down into how much a female will cost. So you get 50/50 ratio on average so you get 5 fems out of a pack of 10 at 50 so thats 10 per seed, say you got some feminized got all fems then thats only 9 per seed, so really per female its cheaper to buy fem seeds.
What I would say is that if your going to get some feminized seeds, don't cheap out, some breeders offer fem seeds at low costs and as a breeder they don't rank that highly but say you got some soma feminized or greenhouse feminized then your going to get better quality with fem seeds that were produced in a better environment. Therefore your chances of getting hermies is reduced.