If you sign up for an account, shirts at Thin Red Line are currently just over $20 apiece. These are pretty solid dress shirts, I'd say comparable to T.M. Lewin or another mid-range English shirt. They now offer a slim fit, which is sort of like Brooks' slim fit from what I understand, and two sleeve lengths. They've also softened their collar lining a bit, which was an issue I had with the last round I bought (I was pretty happy with the last round for the price I paid, despite that issue). I bought six, including shipping ($30) the total was $150.
Sadly slim fit, 15 or even 15.5 x 35.5 is unavailable in white poplin, blue pinpoint, blue PoW, pink mini bengal.