If you're doing some kind of discretionary consumer product, than you could proxy sales conversions based on the amount of traffic your landing page receives. Something on the order of 1-2% of total visits will become orders. If you take email addresses on your landing page, then assume a 15-20% open rate for your future newsletter, and a higher amount, maybe 2-5% conversion to sale from those clicks.
Additionally, depending on what type of product it is, you can usually expect your average order size to be around 150% of the cost of one item. For example if you sell t-shirts for $25, your average order size will be around $37, since most customers will by 1 or 2 items.
