Well it depends on what you mean by "almost mint” if it is only a few light scraps way from being new then you might see over a grand for it but we have to remember that it is a YT125, it's probably not on the top of many collectors lists. It was a good find and sure for $400 you got a great deal and it is worth at least a few hundred more.
1984 Yamaha YT 60 Tri Zinger(current project)
1983 Yamaha YTM 200 (rear suspension kit)..... long term project
2004 BMW R1150R
1972 Honda CT 70
2007 Lifan 110