In 5e, even poor people still get 2sp a day. It’s not clear how much it costs to hire someone to cast spells, but it’s either something they could reasonably pay for to cure blindness, or it’s so much that players can make enough money casting spells that money becomes a non-issue even at fairly low levels. Also, that’s not going to work if you want an NPC that is blind instead of was blind until they met a PC who had a spare second-level spell slot.