I bought my carb cap for $15. With the price of a post at $10 and that nipple at $3, I wouldn't be saving much money.
If you have an extra post not being used, this is definitely a good build.
Up Next: Blonde Ale, German Pils
Primary: Pumpkin Spice Latte Stout
Kegged: Galaxy Dry Hopped Fort Point clone, Blonde Ale
Bottled: S'mores Stout, Blonde Ale