Each case needs to be assessed individually as there are many different causes to surgical colic, all with different potential complications and costs. Simple displacements generally have the least complications and best outcomes, where strangulating lesions requiring resection have the most complications post-op.
The horses age, duration of colic, and any complicating factors should be considered. In the current economy, financial considerations need also be taken into account. A simple displacement surgery may run 4-5k, where a small intestinal resection may run into 15-20k. Your veterinarian should make you aware of what you are looking at financially along the way. Generally there are various decision making times. The first is when we can not control the pain medically and the decision to go to surgery is made (based on clinical findings). Second, is when actually on the table and determine what the cause of the colic is. Third is the 24-48 hours post surgery.
It's difficult to decide when and when not to go to surgery with a colic, and each case needs to be decided on an individual account.