Well, if you really want a cheap design, I've done some drawing on a spare envelope... in crayon.
A simpler station could be built, but, given all the furore about the current large opaque box at Euston, how would building another one fit in with that dynamic? And how well would a restricted station be able to sort out passenger numbers?
By all means, tone down some of the embellishments - but, given it is the cheek HS2 turns to the capital, it could really do without being slapped. The best buildings are functional and beautiful - the works of Charles Holden spring to mind, as well as the station at Wemyss Bay.
If an effort can be made to get the thing open, I don't think it'll take long for people to forget the spiralling costs and mismanagement. And, in terms of a more restricted building - surely it's better to go through the pain of overspending in order to soothe issues that may arise, rather than avoiding pain now but having a massive headache later?