Additionally, there are three cards not from Foundations that will help you build a Commander deck: Sol Ring, Arcane Signet, and Command Tower
Provided you're okay waiting a few weeks after launch, you can sometimes buy promo cards for under $10
They have strong sizes to casting cost, a strong ability when they are on the battlefield, and when they die they turn into their Temple, which taps for their mana and then heads back with a tap at sorcery speed but with a harder to do activation
If another effect (or effects) modifies how much damage your creature with a +1/+1 counter on it would dealby preventing some of it, for examplethe player being dealt damage or the controller of the permanent being dealt damage chooses the order in which any such effects (including Uncivil Unrest's) apply
Obviously the spread isn't the same on all cards (this is especially true of low-value cards), so the way you come out ahead is trading ten cards that retail for $1 (but sell for $0.50) for one card that retails for $10 (but sell for $7), or by trading three $50 cards that sell for $35 for one $150 card that sells for $120