Congrats on your find! Treadle sewing machines are awesome. I sew on one and love it.
There are a lot of them out there, but not that many that are still functional or missing pieces like the shuttle. Most people regard them as decorations now, rather than functional machines unfortunately. I paid $60 for my fully-functional treadle machine on Craigslist. The prices range from free to about $200 typically. There are of course rare machines that command more, but on average, a person should be able to get a fully-functional treadle machine for about $100ish, and a decorative non-functional one for about half that. The coffin box type that you have are more desirable. I've seen those going for $250ish and even $300, but it's all about condition, functionality, etc.
The beauty of these machines is that they are wonderful workhorses. Simple design, built to last. Parts are readily available online. A little TLC and it will probably last another hundred years!
Here area some additional resources:
Singer Dating, US: 1-800-4Singer
Singer Industrials Dating/Info., US: 1-877-738-9869
Singer Dating:
http://Singerco.com/support/serial_numbers.html e-mail: TalkToUs@SingerCo.com
And the International Sewing Machine Collector's Society:
http://www.ismacs.net/home.html
