We study the inefficiency of equilibria of resource buying
games, i.e., congestion games with arbitrary cost-sharing. Under arbitrary
cost-sharing, players do not only declare the resources they will
use, they also declare and submit a payment per resource. If the total
payments on a resource cover its cost, the resource is activated, otherwise
it remains unavailable to the players. Equilibrium existence and
inefficiency under arbitrary cost-sharing is very well understood in certain
models, such as network design games, where the joint cost of every
resource (edge) is constant. In the case of congestion-dependent costs
the understanding is not yet complete. For increasing per player cost
functions, it is known that the optimal solution can be cast as a Nash
equilibrium with the appropriate selection of payments and, hence, the
price of stability is 1. 
In this work we initially focus on the price of anarchy for linear congestion 
games and prove that (in the direct generalization of the arbitrary cost-sharing 
model to congestion-dependent costs) it grows to infinity as the number of players grows large. 
However, we also show that with a natural modification to the cost-sharing model,
the price of anarchy becomes 17/3. Turning our attention to strong Nash
equilibria, we show that the worst-case inefficiency of the best and worst
stable outcomes remains the same as for Nash equilibria, with the strong
price of stability staying at 1 and the strong price of anarchy staying
at 17/3. These results imply arbitrary cost-sharing is comparable to fair
cost-sharing as it has a better best-case scenario and a (slightly) worse
worst-case scenario. We also study models with restricted strategy sets
(uniform matroid congestion games) and properties of best response dynamics
with arbitrary cost-sharing.