It depends on what it is and condition. I got mine for $50 from Shopgoodwill.com last year. I got my Collegiate last year for $50 also, from Ebay. I just bought a nice Olds ambassador cornet yesterday from a local from Craigslist for $50. From my following a lot of Ebay auctions most Ambassadors end up going between $75-$150. Trumpets a bit more. Of course model, year and condition are important.
Up the line are the higher level instruments: Reynolds Contempora, Argenta; King Master, Silversonic, Conn Concert Grand, others: Olds Special, Super, Recording, SuperRecording, etc; Getzen Capri, others, some Buescher models, Bach's, etc. can go $300-$800.
The best thing to do is surf ebay and see what is current for the models you are interested in and also use advance search to check completed auctions for various models. Having luck helps too!
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Dave Rafferty
07 Barrington Trumpet, 1972 Reynolds Medalist
55 Besson 8-10 trumpet
55 Roth by Reynolds Cornet
73 Olds Ambassador Cornet
30's Czech small bore Trumpet
Getzen 80 cornet, 33 American Standard cornet
66 Olds Ambassador Trumpet
www.kalamazooconcertband.org