[Grant Bowker]

Defining a "reasonable price" is very hard to do for many reasons.
1. Do you need it now? next week? within a year?
2. Is your objective to have a "perfect" horn, one that looks fine but doesn't work or one that works but looks a bit rough?
3. How many of the item are available (not just on e-bay). For example I don't remember seeing many of the long horn, but you could take an easily available working short/Chev horn and combine it with a long housing from a not working horn to make a working long horn. At least, I think you can, I haven't done it.
4. How much money do you have available for the project. Jay Leno of the "Tonight Show" has enough money to hire several full time staff to look after his collection. Although Bob and Rob give great help with my truck they expect (reasonably) that I give them an equal amount of help in return.
5. You can search the "vintage parts" section of e-bay motors for completed listings using the phrase "Chevy horn" to see what similar horns have sold for. You will have to screen out the unreasonably high bids from people who get auction fever and just have to have the thing right now. Also look at listings that didn't get any bids. This will give an idea of what people thought was too much. Using Chev distributors as an example, I have seen "working" examples sell for 99 cents and others that look similar sell for 50 dollars. I have also seen items sell at what I though was a "too high" price and an hour later seen a similar item fail to get any bids at a much 10% of the first price. I can't explain why this happens.

On edit: one more caution.
6. If the part can be advertised as fitting a Corvette (car, not ship) the buyer will think it is worth at least twice as much as a functionally identical part that is advertised as Chevrloet. I don't know the Ford equivalent but I' sure it exists.