All these prices are from checking AH within the last 3 months. The price range is what I feel is a good price to pay where you won't lose much if any money (may be able to merch, too) along with the best offers I've seen on the AH.
White P hat: 39-42M Have seen sell offers as low as 41m, and buy offers as high as 41.5m.
Red P hat: 27-32M. Have seen sell offers as low as 29M, buy offers as high as 30M.
Blue P hat: 55-65M. Have seen sell offers as low as 59M, buy offers as high as 60M
Green P hat: 40-50M. Have seen sell offers as low as 45M, buy offers as high as 40M.
Yellow P hat: 24-27M. Have seen sell offers as low as 26M, buy offers as high as 25M.
Pink P hat: 24-28M. Have seen sell offers as low as 24M, buy offers as high as 26M.
Black P hat: This one is tricky, but I have not been able to find anyone selling it less than 80M. Highest buy offer I've seen is maybe 65M. One guy was saying I'll buy one right now for 85M cash but I'm not sure if he was serious or not.
Orange P hat: 27-32M. Haven't seen many buy or sell offers on AH that are fair.
Purple P hat: 18-22M. Have seen sell offers as low as 18M, buy offers as high as 20M.
Not sure at all on Cyan but somewhere between 25-40M.
You should be able to score halloween masks for around 7M, all colors. If you're selling try to ask 8 or 9, you might get lucky. Dragon masks... maybe 6-10m tops, red seems to be worth the most and definitely looks the coolest. Unicorn masks..10-20m....good luck selling those though, I wouldn't pay crap for one of those pieces of garbage lol.