What SMG? I accidentally dropped my sniper rifle ($20,000) and picked up back up right away, but next session it was gone. Ended up waiting until I got a mission where the enemy had a sniper rifle (along with trying drop crates) and got the gun back for free.
Your mods should still be unlocked once you get the gun back.
Worst case scenario, it's only $7,000, so sell a Baller at the mod shop and have $2,000 extra.