Below charges should set automatically:
1)if you buy products within $0 to $50 price = $6.00 freight charges
2)if you buy products within $51 to $250 price = $12.00 freight charges
3)over $251.00 = FREE
How to set up them?
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If you want to modify the shipping Rates according to the Price in the Shopping Cart, then you can use the default Table Rates options.
System > Configuration > Shipping Methods > Table Rates
You have to choose Price Vs Destination in condition option.
Please refer a link for the above for the same. http://www.magentocommerce.com/knowledge-base/entry/how-do-i-set-up-table-rate-shipping