When creating Assembly items, you are able to define a custom price, or leave the system to calculate the total price of all the items included in the Assembly package.
You can decide this using the Enable custom pricing tickbox on the Assembly item card:

The above shows an item with standard pricing – when this Assembly item is scanned or selected in POS, the system will use the total of the items included as the price.
If you choose to use custom pricing, tick the Enable custom pricing tickbox. Enter the price that you'd like the total to be and click on Set Price:

The system will weight that price across each item in the Assembly automatically. Occasionally, this weighting will cause slight rounding differences, in which case you can alter the Assembly price of individual items using the blue pencil icon next to the items in the list.

The above example shows the Assembly with Enable custom pricing ticked and a custom price of £80 was entered. When this Assembly item is added to a transaction in POS, the system will use the custom price.