I need some help.  someone asked me to make a cake for a 70th birthday. this is their vision:

A man at a desk looking out a huge glass window to his garden, with a bird cage outside as well.  on the desk is a computer with his company logo on the screen, and somewhere around the desk, all his grandchildrens names.  

They only want a cake for 20 ppl.  Not a big cake for all of that. 

Besides, I'm usually  good at pricing. but this is a ton of work, and of course it's for next week.  

How do i figure out how to charge? I usually go by servings, plus a little if there are extra fondant/gumpaste.  any ideas?

Wow, that's a lot to expect on a small cake. A lot of modeling and I don't think the customer has any idea the amount of work it is. I'd offer parts of there idea, but not all. That's unless they want to pay you big bucks. Only idea that comes to mind is painting some of the elements wanted on a gumpaste plaque. 

I'm with @jchuck ‍, I don't think it's possible to put all that modeling on a cake that would serve only 20 people (assuming they don't want a huge cake with tons of leftovers). If it were my customer, I'd recommend that they scale back the idea to only a few key parts (maybe just the birdcage and the grandkids' names, for instance), and I wouldn't do any modeling (I'm not good at it, haha!). Instead, I'd do a round cake with those concepts painted on the side, or a sheet cake with them painted on top. Play to your strengths - if modeling is your thing, go for it! Or pipe it on, or whatever you're good at. In my mind, though, all those ideas on one small cake is not feasible.

Let us know how it turns out :)

Yes cakingandbaking 

And can you imagine how cluttered the cake would look trying to get everything on top of a small cake!! Some customers don't understand scale. 

I'm with the others.   WAY too much for that sized cake.