Increase in cost of healthy exercise (4)
I believe the answer is:
hike
I believe this is a double definition.
'increase in cost' is the first definition.
(I've seen this in another clue)
'healthy exercise' is the second definition.
(hiking is a form of exercise)
'of' is the link.
(Other definitions for hike that I've seen before include "Travel by Shank's Mare" , "physical exercise" , "Long walk; big price rise" , "Long, hard walk" , "Long march; increase" .)