I have a few sales of 20" recorded in my database. Two happen to be from Bill H himself:
20 Trans 1930 good ebay 21-Oct-07 $199.00 1 bid
20 Trans 1432 good ebay 20-Mar-14 $480.00 27 bids seller Bill H 52-53
20 trans 2011 keyhole ebay 24-Aug-14 $200.00 2 bids seller Bill H
And in the 15" hats department I don't have any Trans yet (although some sales may be in Transit ;) on the way to my database). Here's what I have:
15 small/60s hats 871/874g good ebay 04-May-14 $305.00 41
15 Small Hats 935/960g tiny bell crack ebay 05-May-14 $160.50 23
15 60s new beat 1075/1515g good ebay 05-May-14 $165.60 16
15 Small hats 935/960g good ebay 05-May-14 $160.50 19
I haven't been collecting Zildjian data very long so facts are scarce. But my opinion (you did ask for opinions) based on generalizing from my large sample 602/Sound Creation research is that prices are highly variable. So my ballparks are going to be wide, especially if I don't have sound files or photos to work from. Say an 80% chance that they will sell in the range:
20" 1990g $120-$280
15" hats $150-$300
but you get what you get on the day. You could get higher or lower depending on a number of factors. If you follow the link to my 602/Sound Creation research that gives an overview of the factors which influence achieved price, and the limitations of the research.
Hopefully Drumaholic will show up and give his valuation.