Non-FOIL Azorius Signets are on the move and FOILs have bottomed out after their first spike.
Azorius Signet is the most saught after of the Signets. They are safe from reprint in Commander 2015 because the Commanders are supposedly enemy-colored. This is also grants a reprieve to the other friendly color combinations Signets, among which Dimir Signet is the most interest. So Non-foils are safe from reprint for some time and are currently rising fast, and this is where I would dump my investment, after grabbing a single FOIL copy.
Buy Foils at or under $6.00, buy non-foils at or under $0.75 and sit back. I expect non-foils to double-up within a few months and to have an upward long-term trajectory thereafter. Foils did spike already for this card, but it's settled down and seemingly bottomed out. I can see NM FOILs at $20.00-$30.00 apiece with a couple years' time. Stick with NM copies.
FOILs for Dimir Signet spiked hard about a year ago, too. Grab non-foils of Dimir Signet for under $0.50 apiece and give them a couple years to comfortably rise to $1.50 apiece. Foils are too highly priced for my taste at the moment, though they are worth monitoring for their next potential price spike. Even if it finds its way into a Commander reprinting, I don't see much loss in value due to the fact that these were originally printed as commons, and a Commander reprinting would only include one Signet per deck. Hardly an increase in supply, but potentially a big increase in demand. Foils would see very strong response to a non-foil reprint.
As for ALL the other signets? Buy only FOILS, at or around $2-$3. After a spike, they'll be $6-10. These will undergo similar price jumps in the FOILs that Dimir and Azorius experienced, in due time.