I was looking at a vehicle to buy but the owner said the power steering pulley is broken. Is it still drivable at all? Obviously I will have to fix it but will I be able to drive it home? Is it super expensive to fix? It's in good condition other than that
It sort of depends on the vehicle. On my '97 Subaru "WRX" project, the steel power steering lines rotted out and were very difficult to replace. So I found a shorter v-belt, and now have no power steering. I ended up loving the road feel, but it is some effort to park. My arms are looking more ripped these days.
The same issue happened on my '95 Bronco (I live in salted roads NY - yea me) and the oversize-tires vehicle was nearly impossible to drive. It took me about 20 minutes to travel 3 miles, and I was foolish even in that, as it really wasn't safe. My "ripped" arms about fell off after that adventure.
Depending on the vehicle, you should be able to get a pulley without much expense. Dorman or "Help" brand may sell one. The expense will be in the puller/installer tool if you plan on DIY and don't already own it.
As @Paulster2 says in his comment, it depends on the car - if the pulley is missing or not turning, the serpentine/ancillary belt won't be running correctly (and would probably tangle on everything and shred itself!). Without that belt in place, anything else that is run from it won't work either - which could include the alternator, water pump, etc.
I had a PAS pulley shear a few years ago - in that case, it also drove the water pump, so the car became immediately un-drivable, as it would have overheated in a matter of minutes.
In terms of fixing, you'll need a new PAS pump and a new serpentine belt (and possibly tensioner, again depending on the car). Check the maintenance schedule for the car as well, as it may be worth doing other things while you're in there (e.g. water pump)