Not as much as you'd think; don't undervalue your current trinkets - or overvalue the VP trinket, either.
The base hit is over the base mastery on your Bottle, but it doesn't start as mastery
. You're looking at ~500 secondary stats and an average of a whopping 30 agility (assuming 3s to proc after the ICD (45s bottle, 105s VP) for each trinket - if it's shorter than 3s to proc, you get more average agility from the bottle). Beyond that, the trinket won't be available until friendly (when you've killed a raid boss) so you might want to plan differently. I'd put the trinket neck-in-neck with the neck (hah, hah) for your upgrades (agility gain is higher on the neck, sec. stats higher on the trinket), but I also presume the neck will be a lot cheaper, and you're more likely to keep it after your first few LFRs (any 502 trinket should beat the valor one).
I'd recommend using a different reforge tool to get yourself more mastery for your gear (try shadowcraft.mmo-mumble.com; enforce the expertise cap in options if you so choose - definitely if you also play combat in the same gear).