I have been contacted by a producer who needs to make a video clip for a band. In one of the scenes in the video, 2 band members (dressed as delivery guys) will be carrying a wedding cake in the middle of the street, and the lead singer comes running towards them, they lower the cake for him to jump over the top, knocking over the groom figure from the top tier. A passerby will slip on the groom, and crash into the cake. Cue cake wreck. 

I have offered to make them a fake 4 tier, and an identical real 4 tier, for practicality's sake.

How can I make a 4 tier cake on the cheap?? They will not be eating the cake (unless they want to scrape it off the pavement). I can do things like omit vanilla, salt, whatever. Other ideas? Box mixes aren't cheap here so I know I will just make them scratch like usual. I could do 2 normal layers, instead of torting and filling 3 times, so only 1 layer of filling between 2 cakes. I don't know whether to use BC on the outside or fondant. They will be filming next Friday and it will be about 90-100ºF, and they will be in the street....I have told them to keep the cakes as cool as possible and out of the sun as much as possible. I'm leaning towards fondant, but I only use ganache under it, so more expensive than BC overall. But I think fondant and ganache will hold up much better in the heat.

I won't use SPS since I don't want the person crashing into the cake to get hurt, so I'll use regular dowels.

Any other ideas to keep costs down?

My brand will appear in the video, on the side of the delivery van they will take the cake out of, and on the clothes the "delivery guys" will wear (I will probably lend her some aprons I use in classes).

A"But I think fondant and ganache will hold up much better in the heat."

What about shortening based frosting? Oil based cake instead of butter based? That is, if butter is more expensive.

I constantly hear about people making cake using a box of betty crocker and a can of soda.  Doesn't sound like it would taste good, but if no one is going to eat it that could be a very cheap way of making a cake to be destroyed.  No need to waste eggs, oil, etc

AMaybe call/email around and see if you can find a restaurant or bakery supply store that is getting rid of expired ingredients?

AAre they not paying you for the cake? If not for your labour, how about the ingredients? I know you said no one will be eating it but you still need ingredients to make a four tier cake. Just saying...