Zahavi's (1975, Journal of Theoretical Biology) handicap principle held that the cost associated with a signal is integral to ensuring that signal is accurate, since dishonest signals would be too costly to bear. Grafen (1990, Journal of Theoretical Biology) held that in order for signal honesty to be maintained, the signal must be differentially costly, such that it is more expensive for lower-quality individuals. Getty (2006, Trends in Ecology and Evolution) disputed this: "Here, I show that (Grafen's) most well known and influential result (‘the marginal cost of advertising. should be greater for worse males’) , which is widely interpreted as a proof of the handicap principle, is based on a simplifying assumption that does not generalize to signalling in sexual selection. I show that the general criterion for reliable signalling can be interpreted to mean that higher quality signallers must be more efficient at converting signals into fitness and that, in sexually selected signalling, higher quality signallers can be more efficient in spite of higher marginal costs."
Nevertheless, Biernaskie, Grafen & Perry (2014, Proceedings of the Royal Society) wrote, "It has long been a challenge to test for the relevant costs of animal signals [2,17,18,25,42]. This is because, as stressed above, the modern view of costly signalling theory does not necessarily imply that observed signals will be costly; instead, the key prediction is that a given signal size will be differentially costly for low-quality individuals [2,13,17,31]." The criterion of differential costliness continues to be invoked despite Getty's critique of it. So, my question is, what is the criterion necessary for the maintenance of signal honesty?
Note: This question asks about the maintenance of costly signals that could theoretically be faked, not so-called index signals that cannot be faked.