In Call of Duty: Modern Warfare 3 for the PC, PS3, Wii, and Xbox 360, you unlock equipment as you progress higher in levels.
Here is the complete list of equipment in Survival Mode, and what level they unlock at.
Claymore x5 – $1000 (limit of 10), unlocked at level 2
C4 x5 – $1500 (limit of 10), unlocked at level 3
Body Armor – $2000, unlocked at level 10
Self Revive – $4000, unlocked at level 12
Sentry Gun – $3000, unlocked at level 17
RPG-7 x2 – $2000 (limit of 4), unlocked at level 27
Sentry Grenade Launcher – $4000, unlocked at level 31
Riot Shield – $3000, unlocked at level 37