It is easy to calculate the effective single race rake for a n-leg race by taking the nth root of the return (= 1-rake).

So a 17% Double has a single race rake of 8.89%.

And a 16% Quad has a single race rake of 4.27%.

This is far superior to 16% Win rakes before rounding down.

If you want to get a feel for biases in doubles markets I suggest you check out Betfair Soccer Accumulators. Last time I looked short price combos were overbacked.