Medivacs- Downside; They cost gas. Upside;They Heal
Warp Prism- Downside; Less units it can carry. Upside; You can warp in as much as you want
Overlord- Upside; No Supply and can drop creep. Downside; Requires upgraded to move fast and drop
for every downside that a unit has, it has something positive to make up for it, look at the rest of the game before you pinpoint one unit
Neither Warp Prisms' nor Overlords' secondary effects are anywhere near as powerful as the healing which Medivacs have.
But this is why Medivacs cost 100 gas.
The price of Medivacs is appropriate; it should not be increased.