You can find used Thiel 3.5 & 3.6 often for under a $1000 a pair on
AudiogoN high end audio auctions, classifieds, hifi chat
While not necessarilly my first choice for monitors I think you can definitely get good results mastering on them if they were paired with a good room and a good amp. fwiw - these were what Paul Zinman was using about 5 years ago when I had him master one of my string quartet's CDs.
Best regards,
Steve Berson