I went the super cheap route and went to Goodwill and picked out one of the men's coats there. You can get in tailored. You may have to pay more for a dressage coat though. Expensive isn't always the best.
I had always gone to goodwill, and if you have more than enough time, I would do that. I've spent about $20 on coats, four of them, as I grew from age 6 to age 15. I finally bought a coat for real last season, when I began eventing. Goodwill has always come through for me, as long as I looked early. Just throwing that out there. XD
On the other hand, show coats do vary in quality and the price reflects it. I spent around $100 for mine, consignment, and that's the best $100 I've ever invested in a piece of clothing. =] Consignment/used is the way to go, IMO.