Double glazing alternatives

Hi,
we just recieved our preliminary estimate for our house but have been given a bit of a shock regarding energy rating costs.
On our house we have 3 large sliding doors on the rear of the house, 2 will be facing a little bit north of straight west and the 3rd facing north. This brought the cost up another $4k.
The builder had already quoted us double glazing everywhere else on the house except those doors and it has R6 ceiling, R4 in subfloor and R2.5 walls. Also we are on the eastern slope of a hill so the west sun should drop behind our neighbours pretty early.

Anyone aware of any other options for these doors we could bring up with the builder to get our 6 star?

Do you really need double glazing ?

Do you have suitable drapes/curtains/blinds ? Surely you can get to 6 without double glazing ?

OOT - if it is noise - those simple and cheap rubber linings (Bunning) will reduce noise by about 60-70%. Cost about $10 per window. DG will improve this but $ for $ not sure its needed. Is noise a problem ?

Yes - you could try Low-E glazing - which is a high performance, single glass - generally cheaper than double glazing. What climate are you in - from the sounds of your insulation you are in a fully framed home, so you have no thermal mass. This might be the main issue if you are in a cool/cold climate.
You could ask your energy assessor if they offer any other compliance pathways - a six star rating is only one way to comply. Verification might be a better option for you if your rating is struggling.