Have never come across a shipper basing their charges on item price? Otherwise you could have something light such as jewellery costing as much to ship as an item of furniture, which somehow doesn't quite sound right?
OK, for weight based shipping you need to set up separate zones for each of your defined areas, so in your case you may set up prices per weight for own country and another zone set up for international orders. For round figures let's say an item costs $25 to ship in your own country, you would assign a weight to the item that related to that shipping cost in your shipping table. You can set up shopping cart pro so that weights are not displayed on the item description. All your customer ever sees is a price for shipping that item.
If it helps get your head around the concept, I ship one item which has specialist polystyrene packaging to ship it in. That is an additional cost to me for which I charge the customer. I simply push up the weight of the item into the next weight band on my tables to cover the cost of the packaging.
As with all shipping costs, folks need to remember to factor in the cost of the packaging and even things such as sealing tape and inkjet ink for printing up the labels, as the costs of simple things like that can really add up during the course of a trading year.