Cunard are probably the 'smartest' of the cruise lines in terms of dress.
Some, but not all, passengers dress to the nines every night. Most people dress as if going to a smart restaurant to go to the main dining room and theatre - men are requested to wear long trousers and a collared shirt but no jacket and tie except on Formal/Celebration Nights. Women go from black trousers (could be non-ripped jeans) and a nice top, to (rarely on non-formal nights) the full long dress.
On Formal Nights dark suits or dinner jacket and bow tie or national dress eg kilt with all the trimmings, is expected in the main dining room, with many women in long dress or certainly quite smart cocktail dress. Some people go completely over the top eg with fancy embroidered silk jackets etc (both sexes), which is fabulous to see.
You can wear more-or-less what you want in the buffet and some of the bars, but no shorts, sleeveless tee shirts or sportswear after 6pm.
BUT, travelling on Cunard is a unique experience. The main dining room has a real Wow factor, specially when it is fully of beautifully-dressed people. It would be a shame to miss the whole point of a Cunard voyage for the sake of a trip to the charity shop for a lovely dress or a DJ or dark jacket for OH. And in the midst of it you might regret not throwing yourself into the whole thing when it's the norm - it's not often you get the chance. They even provide ballroom dancing lessons so you could bravely try a little turn in the Ballroom; there are some real experts there to enjoy, but they are very tolerant of shuffling (or stopping-for-an-argument-about-feet) beginners.
As PP said, drinks are hotel prices, but it's part of the experience to sit in a lovely bar with a cocktail. You can take a litre of wine on board to drink in your cabin to keep costs down.
Tips are an issue. Cunard add them as a daily charge and it does add up. You can ask for them to be removed, but when you meet the lovely staff and see how thoughtfully, tirelessly and cheerfully they work for you, for very low wages, leaving their families for weeks on end, you might want to think about saving a little elsewhere in your life in order to be able to leave the tips on. They also go to the people you don't meet who keep things running like clockwork behind the scenes.