Amazon has the Defender Security Swing Bar Lock for Swing-In Doors (Satin Nickel, U 10308) on sale for $2.49 with free shipping on $25+ or with Prime. Allows door to open 2 inches for viewing. Also on sale: 

• Defender Security U 9590 Lock and Door Reinforcer for $6.52
• 2-Pack Defender Security U 9517 3-1/2 inch Chrome plated Back Plate for $3.10
• Defender Security U 9488 Adjustable Door Strike for $1.40