In the products table for the tee shirt, you enter the base price for the product in the price column, 20 for this item.
In the productsoptions.OptionPriceIncrement you enter how much the selected option should increase the price.
where the sizes S-XL increase the price by $10, enter 10 for the OptionPriceIncrement for those options.
there will be a how to article released next week that will walk through using this in eCart.