Home Repo sez... "Not Available"... So I'll assume you linked to standard "pink fluffy" insulation or similar... If not, this might not apply...
Regular pink fluffy is about 0.8 lb/sq/ft. 703 is 3.0 lb/sq/ft.
So, to have the equivalent of 703, you'ld need to find a way to compress the pink fluffy stuff by a factor of 4.
4 x $3.50= $14.00 sq/ft in pink fluffy vs $8.00 sq/ft for 703 (which $8 sq/ft for 703 is waaaay too much)
Try
SPI for 703 pricing.
[edit]
Just had an afterthought... yeah, I know... scary, huh???
If you make a superchunk style corner trap, you'ld play hell getting the pink fluffy compressed.
But, if you make a 703 cross corner trap, you can back fill the trap with the pink fluffy to add a bit more of a wider "Q" to the trap... thereby extending the absorption upwards in center frequency.