You may be baffled by the huge gap between the estimates. There may be one that's about what you were thinking, and two others totally different. Most people might choose the lower price. (see the article we published previously).
What I didn't address earlier is that the price you want, while it may be just what you were thinking, may deliver not only a poor quality repair or update. It also may not last for very long. We have had to pass on jobs due to customers wanting a quick fix. I've actually recommended a complete chimney tear-down and rebuild when a customer wanted to have just a few courses repaired.
When you're dealing with brick and stone masonry, ask your contractor or potential contractors how long they will guarantee their work to last. This is a quick way to determine what to go with.