Class Rooms Reservations

Hi,
I need to put a class room reservations form application working asap. Have several problems... and i don't know how to solved it...
1- My school have a new financial accounting system that uses internal cost centers and now each class room have a different cost per hour. My live now is only numbers, values, maps of control... a Caos and a eternity to find some errors...

2 - So for my "angel" form i will need that the user select de class room, then select the start hour and the end hour. Then have to point if he will need a datashow, plus a pc, plus water, etc.. (each item with individual cost).
3- Now the problems.... it is possible to calculate the cost of the class room according the number of hours selected and showing it before the submit of form?

4- I would like to have a decison box that have to be un-checked by user to unable my addition itens for support in class room. It is possible?

5 - I spent last days exploring this application but still in very very basic level...

My form is in portuguese language (PT) in:
http://creator.zoho.com/ccongressos/reservas/

Please help me!
Marques Silva (ccongressos)