Possible Improvements, in order of cost:
0 CMC
Shefet Dunes: ~$0.35 each
Tectonic Edge: ~$1.65 each
Mutavault: ~$13.85 each
Aether Vial: ~$30.15 each
Cavern of Souls: ~$58.20 each
1 CMC
Gryff's Boon: ~$0.35 each
Student of Warfare: ~$4.10 each
Kytheon, Hero of Akros: ~$4.35 each
Path to Exile: ~$7.15 each
2 CMC
Declaration in Stone: ~$0.75 each
Hanweir Militia Captain: ~$0.75 each
Return to the Ranks: ~$1.05 each
Knight of the White Orchid: ~$1.10 each
Grand Abolisher: ~$5.40 each
Thalia, Guardian of Thraben: ~$17.85 each
3 CMC
Silverblade Paladin: ~$0.60 each
Mirror Entity: ~$0.90 each
Always Watching: ~$1.00 each
Thalia, Heretic Cathar: ~$1.65 each
Ajani, Caller of the Pride: ~$6.90 each
Gideon of the Trials: ~$15.55 each
4 CMC
Dusk // Dawn: ~$1.35 each
Gideon, Champion of Justice: ~$4.10 each
Gideon, Ally of Zendikar: ~$9.70 each
Ajani Steadfast: ~$11.30 each
Elspeth, Knight-Errant: ~$14.15 each