I searched a lot but was quite surprised to find no info on this.
In Prestashop 1.6.1, the specifc prices do not have any influence on the unit prices which makes the whole specific price mechanism useless for unit price based items.
Example:
We have products with unit prices (e.g. bell peppers), combinations (yellow, green, red) AND specific prices for differnet customer groups (end customer, wholesellers)
We want end customer to have prices
yellow: $20/kg, red $25/kg and green $30/kg
We want whole sellers to have prices
yellow: $15/kg, red $20/kg and green $25/kg
Now for the product price, this works fine when using combinations for the attributes (yellow, red, green) with fixed amount price increases for each combination (+$5). And we can set specific different fixed prices for our wholesale customer group (not ideal but it works)
So far so good.
Problem is, that we cannot set any alteration on the unit price for each customer group. The unit price will always stay the same as defined in the main price tab in product edit page in admin area, no matter what customer group views it.
That basically means it is impossible to create different price rules for products with unit prices in prestashop which I would consider a bug.