You can still get a used Singer "New Family" model for about $120 in today's money on eBay.
But for $350 you can also get a whole new sewing machine [4]. The difference is that today's average wages in the US have climbed to around $1000 per week [5], so you only have to work about three days to afford a sewing machine.
[4]: https://www.singer.com/machines/computerized
[5]: https://fraser.stlouisfed.org/title/monthly-labor-review-6130/june-2022-619258/wages-converging-2010s-expansion-633526?start_page=22
Sorry, I was only able to find one US catalog of Singer, so that's what my calculation is based on. But I suspect that the situation might be similar for other manufacturers and other (developed) countries.
On a side note, I found out that you can still buy newly manufactured replacement parts for treadle sewing machines – but then that's no surprise given that these models still have a large user base in countries with insufficiently developed electrical grids.