You don't have to agree and not all of these ideas are mine.

For the Sentry
+30% Damage
+20 health every time it's upgraded
+50% Increased range
+25% Ammo
+Damage falloff decreased by 35%
-50% Rockets
-Takes slightly longer to target someone
-Rotates 15% slower
-20% Firerate
-Takes 25% Longer to build

For the Dispenser
+50% More healing
+20 health every time it's upgraded
+20% More ammo given
+Depending on the class it will give a 5% attack buff (Offense), a 5% defense buff (Defence) or an extra 5 health regen (Support).
-50% Less Metal stockpiled
-It takes 35% longer to generate metal
-The metal is worth 10% less
-It takes longer for ammo to be given
-Takes 15% Longer to build

For the Teleporter
+20 health every time it's upgraded
+Gives players a small buff (similar to the dispenser) for 5 seconds after leaving the teleporter
-It takes much longer to teleport someone, ex: L1 takes 20 seconds, L2 takes 10s and L3 takes 5s
-Takes 20% Longer to build

For the Engineer
+10% more damage done from weapons in the shotgun and pistol slots
+5% more metal from packs
-15 health
-Move speed decreases by 10% per level when carrying the sentry, dispenser or teleporter