"Tha Phra Athat Pier" is the closest embarkation point for Khao San Rd and depending where you are staying would be a maximum 15 minute walk. Be aware the entry point is a narrow alleyway. At 15 Baht per person to the "Grand Palace" (Tha Chang Pier) it is very cheap. You can pay on the ferry although they seem to prefer it if you paid at the ticket office. Look for the ferry with the Orange flag as that has the best coverage.The flags indicate the service.There are other company's but they seem to have very limited coverage plying their trade between 1 to 2 points on the river. Note:Tha Phra Athat Pier seems to be cheaper (1500 Baht) if you wish to privately hire a Longtail Boat to do the Thornburi canals. I highly recommend a 1.5 hour tour.Further down the river appears to be a lot more expensive.
If you own or manage Chao Phraya Express Boat, register now for free tools to enhance your listing, attract new reviews, and respond to reviewers.