orange = added GST back on (bug 2865) green = correct yellow = wrong
RESULTS
Test Scenarios Expected Totals Total in 3.0 test
Title ordered Supplier Ordering price includes GST discount $USD Conversion rate Unit list price Budgeted known bug 2865 RRP Budgeted RRP
1 copy Electricity and magnetism Technical Books GST 15% $30 $22.67 $25.50 $30.00 $25.50 $30.00
Power to the people Capital Books GST No $30 $26.67 $30.00 30.00 $30.00 $33.75
Tokohia South Pacific No 35% $30 $19.50 $21.94 $33.75 $19.50 $33.75
Your life online Library Supply No No $30 $30.00 $33.75 $33.75 $30.00 $33.75
Settlement by sail Baker and Taylor USD 35% 0.5325 $30 $36.62 $41.20 56.34 $10.38 $15.98
Cushla and her books Amazon USD No 0.5325 $30 $56.34 $63.38 56.34 $15.98 $15.98
       
2 copies Cloncurry story Technical Books GST 15% $30 $45.33 $51.00 $30.00 $51.00 $30.00
Alan Duff's maori heroes Capital Books GST No $30 $53.33 $60.00 $30.00 $60.00 $33.75
Statistics South Pacific No 35% $30 $39.00 $43.88 $33.75 $39.00 $33.75
Guide Rangi Library Supply No No $30 $60.00 $67.50 $33.75 $60.00 $33.75
Volunteering and ethnic comnunities Baker and Taylor USD 35% 0.5325 $30 $73.24 $82.39 $56.34 $20.76 $15.98
Babies need books Amazon USD No 0.5325 $30 $112.68 $126.76 $56.34 $31.96 $15.98
Findings
Worked perfectly for both budget and RRP
1 copy Tokohia South Pacific No 35% $30 $19.50 $21.94 $33.75 $19.50 $33.75
1 copy Your life online Library Supply No No $30 $30.00 $33.75 $33.75 $30.00 $33.75
2 copies Statistics South Pacific No 35% $30 $39.00 $43.88 $33.75 $39.00 $33.75
2 copies Guide Rangi Library Supply No No $30 $60.00 $67.50 $33.75 $60.00 $33.75
Added GST back on to the budget price (bug 2865) but calculated the RRP perfectly
1 copy Electricity and magnetism Technical Books GST 15% $30 $22.67 $25.50 $30.00 $25.50 $30.00
2 copies Cloncurry story Technical Books GST 15% $30 $45.33 $51.00 $30.00 $51.00 $30.00
Added GST onto both the budget price (bug 2865) and the RRP
1 copy Power to the people Capital Books GST No $30 $26.67 $30.00 30.00 $30.00 $33.75
2 copies Alan Duff's maori heroes Capital Books GST No $30 $53.33 $60.00 30.00 $60.00 $33.75
Miscalculated the currency conversion
1 copy Settlement by sail Baker and Taylor USD 35% 0.5325 $30 $36.62 $41.20 56.34 $10.38 $15.98
1 copy Cushla and her books Amazon USD No 0.5325 $30 $56.34 $63.38 56.34 $15.98 $15.98
2 copies Volunteering and ethnic comnunities Baker and Taylor USD 35% 0.5325 $30 $73.24 $82.39 56.34 $20.76 $15.98
2 copies Babies need books Amazon USD No 0.5325 $30 $112.68 $126.76 56.34 $31.96 $15.98
Conclusions
1 Where a Vendor profile has been set with list (ordering) price as not including GST the budgeted price and replacement costs are calculated perfectly, regardless of whether a discount applies or not, and regardless of whether 1 or multiles copies are ordered.
2 Where the Vendor profile has been set with list or ordering price as including GST, and a discount applies, then Koha is correctly removing the GST, correctly removing the discount and then INCORRECTLY adding GST back on again. Ordering 1 or multiple, and varoious discounts are handled fine.  
3 Where the Vendor profile has been set with list or ordering price as including GST but NO discount, Koha is adding GST on top of the both the list and the RRP price. Ordering 1 or multiple copies makes no difference.
4 Foreign currency conversions a mess. Will bug this seperately.