I have not travelled on Caronia but I assume the prices will be similar to QE2.
I was on QE2 last month and the tuxedo prices in the brochure (with shirt tie, cummerbund/waistcoat, hankerchief - no shoes) are as follows :
1 night : $75
2 nights : $115
3 nights : $150
4 nights : $190
5 nights : $225
To me these prices seem high, a lot higher than some of the other lines I have been on.
At hire shops in Scotland you can hire a dinner suit or highland dress for a whole week for the price of two weekends if you ask nicely. This would be about £60-£80 ($90-$140).
I suggest buying a dinner suit anyway, for a good deal try : www.slatermenswear.com . £100 for average and £200+ for quality.
I do not know what choice there was for ladiesware.
I agree with Jon, it does seem expensive. I paid £60 from a local (Surrey) hire shop for two weeks last month. The annoying thing about this is that the Cunard brochure refers to formal dress hire as being "at a nominal charge", $75 seems far from nominal to me. Sorry that I can't comment on the ladies wear as my wife had a great time buying new stuff.
Thanks for your help chaps, but the word from Cunard is that the Caronia doesn't do formal hire at all. Thank you both for the advice about menswear though - I shall be dragging my fellow around the gentlemens outfitters shortly.