A handicap is provable waste. The advantage of a handicap is that it is easily assessed without context. Not all costly signals are handicaps. Payments are not handicaps, therefore they do not have the same advantage as proof-of-work. More context is required to evaluate them. For example, the payment might have been given back in another tex; thus it may not be as much as it appears. Additionally, the value of money is speculative so the cost of a payment is not necessarily something that people all agree on as easily. Furthermore, if you demand payments to have your attention attracted, there is no reason to believe that it actually worked. You could have accepted the payment without reading the message. With proof-of-work you can't benefit other than by reading the message, thus there is no reason to accept it other than to read it.