How much would it cost to replace ignition cylinder in 1998 toyota avalon?

Where are you located ?

You tube shows how and says it is easy. So maybe 2 hrs at the dealer?

If you can get it turn to the accessory position with the old key, then super easy. Just a new key cyl and key.
If the lock just turns and turns, then it's broken inside the key cylinder housing and that is NOT very easy AND expensive. There was a recall on that housing but could be expired now. This was a very common problem with those.