Hi! I personally like the Mono-blue deck just because it should be able to answer other decks so well. With all the countermagic and the swarms of fliers, it can easily get out of hand quickly. I have very few suggestions, and I'll try to keep them all cheap.
Negate is only $0.25, and seems good here.
Tamiyo's Journal (0$.35) could be nice in a control deck too. Lets you either build up extra cards, or can be saved to find win conditions. Clues also seems like a great option for a deck that plays mostly on it's opponents turns. Gives you a mana outlet in case your opponents don't play anything.
Proteus Staff ($0.35) could be a really interesting option. You can use it on a drake and dig for
Guile or one of your other fatties, or use it on your opponents biggest threats to mitigate damage. Its a nice cheap way to add some chaos to the game, and since the decks are all so cheap, it's unlikely there will be huge quantities of giant unstoppable guys.
If there's room left in the budget,
Isochron Scepter ($5.50) could be a blowout.
Counterspell,
Negate,
Anticipate,
Brainstorm,
Impulse,
Vapor Snag... all amazing targets. Plus the Scepter makes you cast the spell instead of just play it, so it will trigger the commander each time. There may not be enough targets, but its something to think about.
Hope you found this helpful! Good luck and Happy Hunting