However...and I may well be in the minority here...except for display purposes, I am not a fan of "dummy" cakes or "dummy" tiers used in place of a real cake. I'm a fan of realism. In other words, if you have only 50 people then have a cake big enough to feed 50 people, not a 5 tier cake that only has two real tiers. If you can't afford anything but sheet cakes then skip the cutting of the cake, have your cake brought out as a dessert and feed eachother from your shared plate. My own opinion of course, but as a wedding coordinator I see high expectations and low budgets all the time and spend a good deal of my working time with a client trying to gently make her expectations meet her budget. You can't have a Ron Ben Israel cake on a total wedding budget of 5K....so your expectations should be tailored for the budget, not try to make the budget stretch for your expectations.
For those of you who have successfully made these rental cakes a hit I congratulate you! You fill a niche for people who need it and yet still inspire those who still want a real cake.