Does personal hotspot cost on iPhone 5?

I want to use my personal hotspot to connect a Wi-Fi to my laptop in the car. My iphone 5 is on an unlimited Internet contract, will it cost extra or is it free?

It's also called Tethering (wif tethering)
and many plans do not allow it

so depends on your plan.
Also note that most unlimited plans are not really unlimited, but have an upper expected limit
and above that, slow down exeedingly in speed - but still there. (O2 does this)

The only plans with unlimited and tethering are Contract plans (not PAYG)
Three's One PLan
T-mobiles Full Monty plan

o2 allows tethering but there's an upper limit of 3GB unless you bought a higher plan
Vodafone allows tethering, but again expect consequences if you go over the FUP limit
Orange allowes tethering, but I did not think they did an unlimited plan

All other Three unlimited plans (except One Plan) are unlimited phone use not tethering, you need to buy the tethering option.