It will cost you 10K Kips, payable at the start and you can also buy some drinks there. It’s 30-40 minutes of uphill to the top (650 meters), often steep and over rocks.
The paths are easy to follow, with occasional red spray-painted arrows on the rocks, but they are steep in places. Ropes are occasionally there to help you grab onto something, but you should have sensible walking shoes on and water with you. I went on a hot dry day, but I can imagine that this would be very slippery in places once wet. Also, be careful about going down. One of our tourist went a little too fast, slipped and hurt his ankle.
Once you reach the top there are several viewing platforms which provide incredible views of the surrounding limestone karsts and patchwork of rice fields. Bring about 1 litre of water per person to stay hydrated on your climb up. There’s a stall selling drink at the viewpoint (not cheap, a can of coke cost 15k (Equiv USD2).
To get here, just follow the Google Map. Generally, you just need to cross the wooden bridge (for bicycle is 6000 Kips, if you’re walking is 4000 Kips). You just need to turn when you see the sign for “big pha nergn view point”.