So I was asked to make a cake topper similar to this...

So my initial thought is to use styrofoam and possibly RKT for the wall portion so that everything is kept super light so that when it is placed on the cake it doesn't cause any issues.  For the base I was going to use a rectangular piece of styrofoam but I am blanking on how to cover the bottom (the part that will touch the cake) so that styrofoam doesn't get mixed into the icing.  For the water portion I was going to use blue fondant mixed with tylose (going to make this possibly this weekend and it isn't needed until Nov 3rd so it will have plenty of time to dry) and then the day it will be picked up add the blue piping gel.  This wouldn't cause any melting issues right?

Any advice on how you would tackle this?  I am not sure about the overall size because I still need to find out if my friend is getting a full or half sheet cake but it will be pretty large regardless.

I know price is super subjective but what would you charge for something like this?  I know the materials won't cost me that much so it is mainly my labor that will determine the price but I don't even know what would be considered normal.

put a cardboard underneath it -- glue it on -- $125 or so more if it's even bigger -- if you are of a mind to use styro -- i would use styro for the wall too -- as you're making it be sure it won't tip over that way -- onto the wall -- i wouldn't worry about the weight -- just toss a couple straws into the cake below to help with the weight -- your water sounds cool -- i would use corn starch over tylose myself -- and if you use styro -- you can stick the thing down onto a bamboo skewer stuck into the cake to ensure it won't tip in transit -- and for something like this i would put a layer of foam under the cake itself so the bamboo skewers have something to attach to underneath -- i often thread my bamboo skewers through the support straws --

can't wait for a picture

Nothing wrong w/using K8's suggestion of cardboard - here's other options : fondant/gumpaste/melted chocolate - take your choice :) 

Ditto what -K8memphis and kakeladi said.  I also agree that styrofoam would be a good choice for the whole structure.  I made a swimming pool cake once and used piping gel for the water and it didn't change the fondant at all and it was fresh (the fondant, I mean).  

Thanks for the input!  I am still on the fence about whether or not to take this order.  My friend needs it about a day after I get back from vacation which means I only have this weekend to work on it and I am currently in a "I don't feel like doing any cake stuff this weekend" mood.

But if I do take this on I will definitely go with all styrofoam.  It would be nice to work on something that I don't have to be too careful with.

Just wondering, do you cover the Styrofoam in fondant or chocolate? If so, do you use buttercream as glue? I never heard of using Styrofoam in cakes! I would think rice crispy treats or cakes of different sizes. Good to know! 

BakedbyMommy, I use Styrofoam as the base for a lot of my sculpted figures because they will never be eaten.  In this case, it would be a great choice because the recipient will more than likely choose to keep the topper.  All of my sculpted pieces are covered with modeling chocolate, but I have used Styrofoam cake separators that I covered with fondant.  Nothing is needed to stick modeling chocolate to Styrofoam.  It holds on quite well.  I did use a bit of glue made with Tylose and water for the fondant covered pieces.

In addition to keeping the piece, styrofoam is nice and light weight which when making a large topper or sculpted piece is ideal so it won't add a ton of weight to the top of the cake.