This is assuming that neither of the units has been upgraded and neither is being healed. Also assuming that all of the marines attack at once and it is a fight to the death.
Using a unit tester, assuming both groups are moving towards each other using attack move, no upgrades, no microing, and that all marines are grouped up, the max that I could take out was 3 marines with 1 zealot with 7 hp left, if all 3 marines are spread out with some small space between each unit, zealot survives with 2 hp most of the time.
A little marine micro and the zealot will die to 3 marines with 1 or 2 left.
4 marines constantly destroy the zealot.
So the problem with this equation is that Marine DPS increases proportional to the number of marines. However, Zealot DPS increases proportional to the surface area of the Marine "ball." So while 1 Zealot can take down 2-3 Marines, 200 Marines will crush 100 Zealots.
If you can find yourself a nice enough choke, you can probably handle 100 Zealots with 150 Marines.
So there is no good rule of thumb here for ratios of Zealots to Marines.
Marines are cost efficient with the proper micro against any number of Zealots, as long as the Zealots do not have charge.
This is because: 1 Zealot = 2 Marines and 2 Marines kill 1 Zealot. Just run circles with the Marine which is chased by the Zealot around the other one. If the Zealot changes his target, just switch the roles of your Marines.
The same goes for any amount of Zealots, as the Marines will always double their numbers. At some point it is not possible to use the merry-go-round tactic as the amount of Micro increases too much. Then you have to stutter step and focus some of the Zealots as shown in this video @ 2:40.
So the answer to your question would be: less than 2 Marines.