Depends what kind of kit and whether you're installing an off the shelf fit for EMS or custom.
Hint - off the shelf is cheaper.
Anywhere from $0 (rip seats out and use hardpoints to lock down stretchers) up to $1m depending (full EMS including rotating stretcher points and defibs etc).
Also depends a lot on the ac type. Some ac (EC135) are basically born to do this kind of work. Moding an S92 would be harder because of size/ scale.
As whoknows alludes to, depends on your exact requirement and what you/ your customer wants as to how 'long' your budget needs to be.