Hey Xavier,
I haven't really checked eBay tape prices in quite a while so I'll just generalize with prices.
The 1/2 track tapes that I have so far have been all over the place as far as sound goes. But one rule of thumb is that the ones that were produced in the late 50's to the mid 60's are much more consistent. Recordings like some early production Mercury tapes sound great but of course, they're very "hissy". Recordings with Paray/Dorati/Hanson at the helm of their respective orchestras can really be a joy. These performances and recordings by Wilma Cozart Fine make it easy to forget about the hiss and just get lost in the music.
Likewise for RCA releases with Reiner.
The above tapes are the only ones that I currently feel are worth paying high prices for (your budget will determine exactly what high is). There are some exceptions of course.
There is a personal fondness for tape and the sound it produces that is at least a 25% factor in my thinking.
The very best vinyl reproduction is different in it's presentation. Some of this is because, as Tom Fine noted in the thread regarding tape duplication, Mercury actually made their running masters from the session tape eliminating an entire generation (as Paul does with the Reference Recordings tapes and I'd venture to guess, the Jacqui Naylor also).
These are some of the things that I'm trying to find out and will post it in that same duplication thread. I'm currently trying to contact someone at RCA to find out first hand, what their practices were and how they evolved as time passed.
At this time, I haven't really gone crazy trying to obtain these tapes. I've still got at least 50 Barclay-Crockers to get to complete that collection and I can only afford to pursue that at the moment.
If you haven't already checked out Charles King's wonderful ratings of the 2k or so 1/2 track tapes that were made, go to the Yahoo Groups Reel to Reel group and look for it in the saved files section.
A few words of warning to the 1/2 track beginner;
The early Mercuries and RCA's aren't without a few "dogs" in the bunch.
And these early tapes are all on acetate. That means that they are "delicate little flowers" and break very easily. There are precautions that should be taken and special handling procedures to be followed (these can be found in the "beginners guide").