You are charging them. That's why they only get 10 points back for losing 50 points worth of attributes.
PACKS costs are the same as normal character creation costs. To see that, let's compare PACKS to normal character generation, using the Brawler attribute kit, which goes like this:
Body 5, Agility 4, Reaction 3, Strength 5, Charisma 3, Intuition 3, Logic 2, Willpower 3. The kit costs 200 BP.
Then we decide to make this character a troll, and we get the following scores:
Body 9, Agility 3, Reaction 3, Strength 9, Charisma 1, Intuition 2, Logic 1, Willpower 3. We get back 10 BP, so the total cost is 190 BP.
Now let's do character creation the conventional way. We choose the troll metatype, which costs us 40 BP. As a result, we have the following base attributes:
Body 5, Agility 1, Reaction 1, Strength 5, Charisma 1, Intuition 1, Logic 1, Willpower 1
To get to the same level as the character we made with PACKS, we have to make the following adjustments:
Body +4, Agility +2, Reaction +2, Strength +4, Charisma --, Intuition +1, Logic --, Willpower +2.
Combining all those pluses gives us +15. Since attribute costs are 10 BP per point, that means the total costs of making those changes is 150 BP. We add that to the 40 BP cost of choosing to be a troll and get 190 BP, which is the exact same cost that we got through doing it with PACKS.
The elegant thing about PACKS is, nothing in the rules changes. It just does a lot of the math for you so that you can do things quickly and easily. But whether you use PACKS or whether you generate the normal way, the BP cost remains the same. PACKS, then, is not doing anything with BP costs that the SR4A rules didn't already do.