This page might help clarify the pricing:
http://www.opticscentral.com.au/layby-system.html
Near as I can tell it appears that their layby pricing is 15% of the total price. In the case of this particular 'scope that would make the total $600.
That said I do believe the way they've elected to advertise their layby pricing is pretty obscure, and I believe could be construed as deliberately misleading. Obviously the $90 pricing is designed to grab your attention (along with the other items on the page link above).
Whether the company is legally obligated to also make the TOTAL price clear (as mobile phone plan pricing must do) - I don't know.