More accurate to say he was a noble during baulders gate 2. Now he is a vampire who was forced to feed of rats, so he is not really rolling around in gold. Most likely that armour he stole of a guard he killed when he escaped Raphael / gale. [read my post on steam forums about Raphael being gale if your curious].
So it makes sense that his armour would give him disadvantage. Proper stealth gear is highly expensive, and rarely made of leather at all. Normally cloth with very specific weave of chain mail that was imbedded in double layer of very thick fur to remove sound via sound isolation. Likewise my impression of Astarion is that he lost all his gold anyway.
