This is actually a copy of a comment I made a while back, but it never saw much light and it was sort of hidden, I am reposting it as a main page to make it more useful. hopefully it helps 
Going to do a REALLY rough approximate here,
-Scouts- call it 4 gauss shots, 38.5 Dam per shot. assuming no resistances we can say around 154 HP
-Cyclops- I have noticed 2 shots from 4 Gauss cannons exert (20% health) although they do heal, we can approximate that it is 308 (4x38.5x2) for each 20% or 1540 HP in total
-Basilisk- I have noticed takes 4x4 shots to exert. again ignoring healing and resistance. (4x38.5x4) gives us 616 and times that by number of heart sequences being 6 gives us 3696 HP in total.
-Medusa- Takes 6x4 shots to exert following the same method as above (4x38.5x6) is 924 per heart sequence with 7 heart sequences (including the end part when they have no hearts) is 6468 HP
-Hydra- takes about 8-10 shots (sorry, only tried the few times and never got the heart) so using the method above times 9 heart cycles we are looking at a number between 1232-1540 per heart sequence, and 11088-13860 HP. the Hydra has a crazy regen on it though, so it is likely a touch less.
On a odd note, with those figures, It is actually the Basilisk that pays the most per HP. They medusa and hydra actually taper off on pay whilst the Cyclops is only worth 1/3 for half the HP.
As I said, a very approximate list, hopefully someone can refine in a better way in the future!
Going to do a REALLY rough approximate here,
-Scouts- call it 4 gauss shots, 38.5 Dam per shot. assuming no resistances we can say around 154 HP
-Cyclops- I have noticed 2 shots from 4 Gauss cannons exert (20% health) although they do heal, we can approximate that it is 308 (4x38.5x2) for each 20% or 1540 HP in total
-Basilisk- I have noticed takes 4x4 shots to exert. again ignoring healing and resistance. (4x38.5x4) gives us 616 and times that by number of heart sequences being 6 gives us 3696 HP in total.
-Medusa- Takes 6x4 shots to exert following the same method as above (4x38.5x6) is 924 per heart sequence with 7 heart sequences (including the end part when they have no hearts) is 6468 HP
-Hydra- takes about 8-10 shots (sorry, only tried the few times and never got the heart) so using the method above times 9 heart cycles we are looking at a number between 1232-1540 per heart sequence, and 11088-13860 HP. the Hydra has a crazy regen on it though, so it is likely a touch less.
On a odd note, with those figures, It is actually the Basilisk that pays the most per HP. They medusa and hydra actually taper off on pay whilst the Cyclops is only worth 1/3 for half the HP.
As I said, a very approximate list, hopefully someone can refine in a better way in the future!