So I have an iPhone that's connected to wifi. I forgot the Wifi password and want to connect a device to my phones personal hotspot, does it cost extra money if I do?
That depends on your service provider. In the US if you use T-mobile with an unlimited plan the answer would be no.
If you pay for data the answer would be yes. The hotspot on your phone is set up to use your mobile data.
One way or another, you pay for tethered data based on what service provider you choose and what plan you choose.