If you just want a PDF version there's no more to pay. As soon as the book's complete you'll get a code to add the PDF to your DriveThru account (you may have to set one up if you don't already have one, it's pretty painless). You can access it from there, save it to your computer and mobile devices and so on.
If you want a print version then once the book's complete you'll get a code which will allow you to buy the book at DriveThru at cost price, meaning the printing and shipping fees from the publisher. I haven't got an exact figure for these costs because I don't know how long the book will be and what print prices are going to do in the meantime but using 450 pages for a ballpark figure the extra print costs are:
black and white softcover - $8.72 US or £5.92
colour softcover or black and white hardcover - $14.61 US / £10.02.
colour hardcover - $20.22 US / £13.65
Cards will be roughly $8 assuming 100 cards, $9 with a plastic deckbox or $10 with a custom tuckbox.