I had never heard of the cover your plate idea until reading EHell. Growing up, the price of your gift was based on 1)how close you were to the couple 2)your financial situation. It had nothing to do with whether you attended the wedding or not or the cost of the wedding attended.
If you were wealthy Great Aunt Martha, you were probably going to give one of the more expensive china pieces, whether she attends the wedding or not. Aunt Jill who has 2 kid at home and another in college is probably going to buy a place setting and Jill, her DH, and the 2 at home kids and maybe the college one will all be in attendance. Cousin Adam who graduated college last year and was in his first year of teaching, is probably going to give a place setting of flatware. And Sheila who you volunteer with one Sat a month is probably going to give a nice picture frame. Mom's next door neighbor may gift a set of fingertip towels she has hand embroidered. The expectation that these 5 guests should all give a gift of the same monetary value because they all attended the same function is just very foreign concept to me.
I understand a couple who grew up in a culture where every one gave a monetary gift equal to the per head cost having that expectation, but only expect if they only invited guests who are members of that culture. As a host you should not expect to force your cultural expectations on everyone else. If you want a "pay at the door" wedding, then only invite people you are comfortable with that model.