How much to pay for a cruising boat
No rules of thumb. A lot of boats on the market are over-priced, and some are over-priced at any price. A few are under-priced. Much depends on the particular boat, its market niche, its condition, and the psychology of the seller and of the broker (and the buyer). In my case, I bought an obsolete 34'' racing boat in need of some TLC from a seller who was "walking away from sailing" through a broker who didn''t really want to be representing the boat in the first place.
You have to decide for yourself just how much you are willing to pay for any given boat, given its condition and suitability for the sailing you want to do with it. Then you just have to come across the right boat in the right place at the right time.