How much do you usually charge for domain parking?

Generally, it is included free with domain registration.
I have not heard of anyone charging for domain parking, as really this isn't even a service. You are just registering the domain and letting it sit.

Or am I wrong about domain parking?

Way back in the beginning there was a charge for it but since the domain business was blown open due to a few changes made by the goverment, etc. this has gone the way of the doo doo bird.

If you can charge for it and get away with it good for you but personally I would not hold my breath. The only small source of revenue that I can see from domain parking is have a simple this domain is parked with xxxx page that has a banner on it. Once again good luck in trying to get some one to advertise on that type of page in today's banner ad market.

Ofcourse if you register and park the domain maybe you could use it to offer free banner ads to other sites that are hosted by you, then ofcouse you have the cost of hosting and serving the banner, unless you require them to serve it.

Some companies do charge for the domain parking. It can run $15/year or more. Many hosting companies are trying to move to a per/mo charge in order to generate revenue. Large hosting companies have tight margins that's why you see this nickel and dime charging going on :(

Most registrars these days even let you park the domains with them so I don't see a point in charging for domain parking unless if you don't want them to be your client.