It's only pitty that many travel agencies in Puerto Princesa grab their customers upon arrival to Palawan, sell them overprised tours. It's an embarasing industry, bring-show/fast photos-leave. Sabang is more than Underground river which is also good experience.
An advice for real travellers. Arrive to Sabang a day before planned river visit, enjoy local village layed-back atmosphere. Stay in Dub Dub native jungle cottages. On the other morning go to the office not later than 8am, register to make sure to escape the crowds coming from Puerto Princesa around 10am. You may consider to get to the place by jungle treking. After underground river you'll have heaps of time to enjoy another things to do around like Mangrove boat trip (P350), beautifull beaches (2 they are, the seccond with not very frequent zip-line is my favourite). Stay another night and after than you might be ready to leave this beautifull place ... or not? Depend on how much time you can put on side from your vacation. Enjoy