The RPG in Roblox Jailbreak is the Rocket Launcher, and its ammo costs Cash Icon 2,000 per rocket in the current wiki listing. The weapon itself is also described as having free, unlimited ammunition in one source, so the practical cost depends on whether you mean the launcher or just the ammo.

Quick read

  • If you mean the launcher: the source I found does not show a purchase price in the snippet.
  • If you mean firing it: each rocket costs 2,000 cash.

What this means

The wording around “RPG” in Jailbreak can be confusing because players often use it to mean the Rocket Launcher. Based on the available source, the clearest verified number is the 2,000 cash per rocket.

TL;DR

The RPG/rocket launcher ammo costs 2,000 in-game cash per rocket, but I could not verify a separate buy price for the launcher from the available result.