Cad-style .Net software as a service

I've been just introduced to cloud computing ans SaaS and it seems very very fascinating! I have actually written a .Net software and it is ready to be published. Now that I have become familiar with SaaS I have got second thoughts about releasing it as a downloadable file. But I have some questions regarding this:

1. 
   

   My program is a CAD-style software so the user will need to interact with an interface? Will it be the interface of my actual program (.exe) file or will they view it on a browser?

   
   


2. 
   

   Will they need to download the whole package of the software onto their PC? Or only the part that they will use to interact with the program? I like the second way cause I don't like to give away the program.

   
   


3. 
   

   How this will affect the performance of the software if the main parts of the program are in the server?

   
   


4. 
   

   I couldn't understand the plans on cloud service websites. What $/hour means? Let's say I have 100 customers and each of them uses the software 10 hours a day. 100x10 = 1000 hours a day. and the price for an hour is $3.5. Does that meanI will have to pay $3500 per day? Isn't that too expensive?